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Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                              P. Hoffman
Expires: February 23, 2014                                VPN Consortium
                                                         August 22, 2013


              Concise Binary Object Representation (CBOR)
                         draft-bormann-cbor-06

Abstract

   The Concise Binary Object Representation (CBOR) is a data format
   whose design goals include the possibility of extremely small code
   size, fairly small message size, and extensibility without the need
   for version negotiation.  These design goals make it different from
   earlier binary serializations such as ASN.1 and MessagePack, or other
   binary serializations that may be created in the future.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 23, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Specification of the CBOR Encoding  . . . . . . . . . . . . .   6
     2.1.  Major Types . . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Indefinite Lengths for Some Major Types . . . . . . . . .   9
     2.3.  Floating Point Numbers and Values with No Content . . . .  12
     2.4.  Optional Tagging of Items . . . . . . . . . . . . . . . .  13
       2.4.1.  Date and Time . . . . . . . . . . . . . . . . . . . .  15
       2.4.2.  Bignums . . . . . . . . . . . . . . . . . . . . . . .  15
       2.4.3.  Decimal Fractions and Bigfloats . . . . . . . . . . .  16
       2.4.4.  Content Hints . . . . . . . . . . . . . . . . . . . .  17
         2.4.4.1.  Encoded CBOR data item  . . . . . . . . . . . . .  17
         2.4.4.2.  Expected Later Encoding for CBOR to JSON
                   Converters  . . . . . . . . . . . . . . . . . . .  17
         2.4.4.3.  Encoded Text  . . . . . . . . . . . . . . . . . .  18
       2.4.5.  Self-describe CBOR  . . . . . . . . . . . . . . . . .  18
   3.  Creating CBOR-Based Protocols . . . . . . . . . . . . . . . .  19
     3.1.  CBOR in Streaming Applications  . . . . . . . . . . . . .  20
     3.2.  Generic Encoders and Decoders . . . . . . . . . . . . . .  20
     3.3.  Syntax Errors . . . . . . . . . . . . . . . . . . . . . .  21
       3.3.1.  Incomplete CBOR data items  . . . . . . . . . . . . .  21
       3.3.2.  Malformed Indefinite Length Items . . . . . . . . . .  22
       3.3.3.  Unknown Additional Information Values . . . . . . . .  22
     3.4.  Other Decoding Errors . . . . . . . . . . . . . . . . . .  22
     3.5.  Handling Unknown Simple Values and Tags . . . . . . . . .  23
     3.6.  Numbers . . . . . . . . . . . . . . . . . . . . . . . . .  23
     3.7.  Specifying Keys for Maps  . . . . . . . . . . . . . . . .  24
     3.8.  Undefined Values  . . . . . . . . . . . . . . . . . . . .  25
     3.9.  Canonical CBOR  . . . . . . . . . . . . . . . . . . . . .  25
     3.10. Strict Mode . . . . . . . . . . . . . . . . . . . . . . .  27
   4.  Converting Data Between CBOR and JSON . . . . . . . . . . . .  28
     4.1.  Converting From CBOR to JSON  . . . . . . . . . . . . . .  29
     4.2.  Converting From JSON to CBOR  . . . . . . . . . . . . . .  30
   5.  Future Evolution of CBOR  . . . . . . . . . . . . . . . . . .  31
     5.1.  Extension Points  . . . . . . . . . . . . . . . . . . . .  31
     5.2.  Curating the Additional Information Space . . . . . . . .  32
   6.  Diagnostic Notation . . . . . . . . . . . . . . . . . . . . .  32
     6.1.  Encoding indicators . . . . . . . . . . . . . . . . . . .  33
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
     7.1.  Simple Values Registry  . . . . . . . . . . . . . . . . .  34
     7.2.  Tags Registry . . . . . . . . . . . . . . . . . . . . . .  34
     7.3.  Media Type ("MIME Type")  . . . . . . . . . . . . . . . .  34



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     7.4.  CoAP Content-Format . . . . . . . . . . . . . . . . . . .  35
     7.5.  The +cbor Structured Syntax Suffix Registration . . . . .  36
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  36
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  37
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     10.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  39
   Appendix B.  Jump Table . . . . . . . . . . . . . . . . . . . . .  44
   Appendix C.  Pseudocode . . . . . . . . . . . . . . . . . . . . .  47
   Appendix D.  Half-precision . . . . . . . . . . . . . . . . . . .  49
   Appendix E.  Comparison of Other Binary Formats to CBOR's Design
                Objectives . . . . . . . . . . . . . . . . . . . . .  50
     E.1.  ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . .  50
     E.2.  MessagePack . . . . . . . . . . . . . . . . . . . . . . .  51
     E.3.  BSON  . . . . . . . . . . . . . . . . . . . . . . . . . .  51
     E.4.  UBJSON  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     E.5.  MSDTP: RFC 713  . . . . . . . . . . . . . . . . . . . . .  52
     E.6.  Conciseness On The Wire . . . . . . . . . . . . . . . . .  52
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  53

1.  Introduction

   There are hundreds of standardized formats for binary representation
   of structured data.  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 serialization is for an
   extended version of the JSON data model [RFC4627].  It is important
   to note that this is not a proposal that the grammar in RFC 4627 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 which starts
   from JSON.

   Appendix E lists some existing binary formats and discusses how well
   they do or do not fit the design objectives of CBOR.

1.1.  Objectives

   The objectives of the Concise Binary Object Representation (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.



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

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

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

       *  An encoder and a decoder need to be implementable in a very
          small amount of code, thus being applicable to class 1
          constrained nodes as defined in [I-D.ietf-lwig-terminology].

       *  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 implementation complexity maintaining a lower
          bound.  Using either general compression schemes or extensive
          bit-fiddling violates the complexity goals.

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

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

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




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       *  It must support a reasonable level of conversion as long as
          the data represented are 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, with the extended data being able
       to be decoded by earlier decoders.

       *  The format is designed for decades of use.

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

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

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119, BCP 14
   [RFC2119] and indicate requirement levels for compliant CBOR
   implementations.

   The term "byte" is used in its now-customary sense as a synonym for
   "octet".  All multi-byte values are encoded in network byte order
   (that is, most significant byte first, also known as "big-endian").

   This specification makes use of the following terminology:

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

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

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







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

   Well-formed:  A data item that follows the syntactic structure of
      CBOR.  A well-formed data item uses the initial bytes and the byte
      strings and/or data items that are implied by their values as
      defined in CBOR and is not followed by extraneous data.

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

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

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

2.  Specification of the CBOR Encoding

   A CBOR encoded data item is structured and encoded as described in
   this section.  The encoding is summarized in Table 4.

   The initial byte of each data item contains both information about
   the major type (the high-order 3 bits, described in Section 2.1) and
   additional information (the low-order 5 bits).  When the value of the
   additional information is less than 24, it is directly used as a
   small unsigned integer.  When it is 24 to 27, the additional bytes
   for a variable-length integer immediately follow; the values 24 to 27
   of the additional information specify that its length is a 1-, 2-, 4-
   or 8-byte unsigned integer, respectively.  Additional information
   value 31 is used for indefinite length items, described in
   Section 2.2.  Additional information values 28 to 30 are reserved for
   future expansion.









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   In all additional information values, the resulting integer is
   interpreted depending on the major type.  It may represent the actual
   data: for example, in integer types the resulting integer is used for
   the value itself.  It may instead supply length information: for
   example, in byte strings it gives the length of the byte string data
   that follows.

   A CBOR decoder implementation can be based on a jump table with all
   256 defined values for the initial byte (Table 4).  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 like).

2.1.  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.  The 5-bit additional information
      is either the integer itself (for additional information values 0
      through 23), or the length of additional data.  Additional
      information 24 means the value is represented in an additional
      uint8_t, 25 means a uint16_t, 26 means a uint32_t, and 27 means a
      uint64_t.  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.  The encoding follows the rules
      for unsigned integers (major type 0), except that the value is
      then -1 minus the encoded unsigned integer.  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 string's length in bytes is
      represented following the rules for positive integers (major type
      0).  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:  string of Unicode characters that is encoded as UTF-8
      [RFC3629].  The format of this type is identical to that of byte



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      strings (major type 2), that is, as with major type 2, the length
      gives the number of bytes.  This type is provided for systems that
      need to interpret or display human-readable text, and allows the
      differentiation between unstructured bytes and text that has a
      specified repertoire and encoding.  In contrast to formats such as
      JSON, the Unicode characters in this type are never escaped.
      Thus, a newline character (U+000A) is always represented in a
      string as the byte 0x0a, and never as the bytes 0x5c6e (the
      characters "\" and "n") or as 0x5c7530303061 (the characters "\",
      "u", "0", "0", "0", and "a").

   Major type 4:  an array of data items.  Arrays are also called lists,
      sequences, or tuples.  The array's length follows the rules for
      byte strings (major type 2), except that the length denotes the
      number of data items, not the length in bytes that the array takes
      up.  Items in an array do not need to all be of the same type.
      For example, an array that contains 10 items of any type would
      have an initial byte of 0b100_01010 (major type of 4, additional
      information of 10 for the length) followed by the 10 remaining
      items.

   Major type 5:  a map of pairs of data items.  Maps are also called
      tables, dictionaries, hashes, or objects (in JSON).  A map is
      comprised of pairs of data items, each pair consisting of a key
      which is immediately followed by a value.  The map's length
      follows the rules for byte strings (major type 2), except that the
      length denotes the number of pairs, not the length in bytes that
      the map takes up.  For example, a map that contains 9 pairs would
      have an initial byte of 0b101_01001 (major type of 5, additional
      information of 9 for the number of pairs) followed by the 18
      remaining items.  The first item is the first key, the second item
      is the first value, the third item is the second key, and so on.
      A map that has duplicate keys may be well-formed but it is not
      valid, and thus it causes indeterminate decoding; see also
      Section 3.7.

   Major type 6:  optional semantic tagging of other major types.  See
      Section 2.4.

   Major type 7:  floating point numbers and simple data types that need
      no content, as well as the "break" stop code.  See Section 2.3.

   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 4).






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

2.2.  Indefinite Lengths for Some Major Types

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

   The indefinite length item is closed by encoding a "break" stop code.
   "Break" is encoded with major type 7 and additional information value
   31 (0b111_11111), but is not itself a data item: it is just a
   syntactic feature to close the indefinite length item.  Indefinite
   length encoding means that the initial indefinite length indicator
   encloses zero or more byte strings ("chunks"), terminated by the
   "break".  A decoding application treats the items that follow, in
   order, as if they were a single larger item.

   For byte strings, there MUST be only byte strings between the
   indefinite length indicator and the "break".  Every CBOR item between
   the byte string with the indefinite length indicator and the "break"
   MUST be a definite length byte string item; if the parser sees any
   item type other than a byte string before it sees the "break", it is
   an error.

   For example, assume the sequence:

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

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


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








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   Text strings with indefinite lengths act the same as byte strings
   with indefinite lengths, except that all enclosed items (chunks) MUST
   be text strings.  Note that this implies that the bytes of a single
   UTF-8 character cannot be spread between items: a new item can only
   be started at a character boundary.

   Arrays and maps with indefinite lengths allow any number of items
   (arrays) and key/value pairs (maps) to be given before the "break"
   stop code.  For example, assume an encoder wants to represent the
   abstract array [1, [2, 3], [4, 5]].  The definite-length encoding
   would be 0x8301820203820405:

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


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

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


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



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      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"


   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        -- -2
      FF        -- "break"


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






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2.3.  Floating Point Numbers and Values with No Content

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

    +-------------+--------------------------------------------------+
    | 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       | (Unassigned)                                     |
    |             |                                                  |
    | 31          | "break" stop code for indefinite arrays and maps |
    +-------------+--------------------------------------------------+

        Table 1: Values for Additional Information in Major Type 7

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

                       +---------+-----------------+
                       | value   | semantics       |
                       +---------+-----------------+
                       | 0..19   | (Unassigned)    |
                       |         |                 |
                       | 20      | False           |
                       |         |                 |
                       | 21      | True            |
                       |         |                 |
                       | 22      | Null            |
                       |         |                 |



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                       | 23      | Undefined value |
                       |         |                 |
                       | 24..31  | (reserved)      |
                       |         |                 |
                       | 32..255 | (Unassigned)    |
                       +---------+-----------------+

                          Table 2: Simple Values

   The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit
   IEEE 754 binary floating point values.  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.)

2.4.  Optional Tagging of Items

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

   The initial bytes of the tag follow the rules for positive integers
   (major type 0).  The tag is followed by a single data item of any
   type.  For example, assume that a byte string of length 12 is marked
   with a tag to indicate it is a positive bignum.  This would be marked
   as 0b110_00010 (major type 6, additional information 2 for the tag)
   followed by 0b010_01100 (major type 2, additional information of 12
   for the length) followed by the 12 bytes of the bignum.

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









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

   Applications may use specific tags defined in the following list and/
   or defined by standard action or in the registry.

   +--------------+------------------+---------------------------------+
   | tag          | data item        | semantics                       |
   +--------------+------------------+---------------------------------+
   | 0            | UTF-8 string     | Standard date/time string; see  |
   |              |                  | Section 2.4.1                   |
   |              |                  |                                 |
   | 1            | multiple         | Epoch-based date/time; see      |
   |              |                  | Section 2.4.1                   |
   |              |                  |                                 |
   | 2            | byte string      | Positive bignum; see Section    |
   |              |                  | 2.4.2                           |
   |              |                  |                                 |
   | 3            | byte string      | Negative bignum; see Section    |
   |              |                  | 2.4.2                           |
   |              |                  |                                 |
   | 4            | array            | Decimal fraction; see Section   |
   |              |                  | 2.4.3                           |
   |              |                  |                                 |
   | 5            | array            | Bigfloat; see Section 2.4.3     |
   |              |                  |                                 |
   | 6..20        | (Unassigned)     | (Unassigned)                    |
   |              |                  |                                 |
   | 21           | multiple         | Expected conversion to          |
   |              |                  | base64url encoding; see Section |
   |              |                  | 2.4.4.2                         |
   |              |                  |                                 |
   | 22           | multiple         | Expected conversion to base64   |
   |              |                  | encoding; see Section 2.4.4.2   |
   |              |                  |                                 |
   | 23           | multiple         | Expected conversion to base16   |
   |              |                  | encoding; see Section 2.4.4.2   |
   |              |                  |                                 |
   | 24           | byte string      | Encoded CBOR data item; see     |
   |              |                  | Section 2.4.4.1                 |
   |              |                  |                                 |
   | 25..31       | (Unassigned)     | (Unassigned)                    |
   |              |                  |                                 |
   | 32           | UTF-8 string     | URI; see Section 2.4.4.3        |



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   |              |                  |                                 |
   | 33           | UTF-8 string     | Base64url; see Section 2.4.4.3  |
   |              |                  |                                 |
   | 34           | UTF-8 string     | Base64; see Section 2.4.4.3     |
   |              |                  |                                 |
   | 35           | UTF-8 string     | Regular expression; see Section |
   |              |                  | 2.4.4.3                         |
   |              |                  |                                 |
   | 36           | UTF-8 string     | MIME message; see Section       |
   |              |                  | 2.4.4.3                         |
   |              |                  |                                 |
   | 37..55798    | (Unassigned)     | (Unassigned)                    |
   |              |                  |                                 |
   | 55799        | multiple         | Self-describe CBOR; see Section |
   |              |                  | 2.4.5                           |
   |              |                  |                                 |
   | 55800+       | (Unassigned)     | (Unassigned)                    |
   +--------------+------------------+---------------------------------+

                         Table 3: Values for tags

2.4.1.  Date and Time

   Tag value 0 is for date/time strings that follow the standard format
   described in [RFC3339], as refined by Section 3.3 of [RFC4287].

   Tag value 1 is for numerical representation of seconds relative to
   1970-01-01T00:00Z in UTC time.  (For the non-negative values that
   POSIX defines, the number of seconds is counted in the same way as
   for POSIX "seconds since the epoch" [section 4.15 'Seconds Since the Epoch'"">TIME_T].)  The tagged item can
   be a positive or negative integer (major types 0 and 1), or a
   floating point number (major type 7 with additional information 25,
   26 or 27).  Note that the number can be negative (time before
   1970-01-01T00:00Z) and, if a floating point number, indicate
   fractional seconds.

2.4.2.  Bignums

   Bignums are integers that do not fit into the basic integer
   representations provided by major types 0 and 1.  They are encoded as
   a byte string data item, which is interpreted as an unsigned integer
   n in network byte order.  For tag value 2, the value of the bignum is
   n.  For tag value 3, the value of the bignum is -1 - n.  Decoders
   that understand these tags MUST be able to decode bignums that have
   leading zeroes.

   For example, the number 18446744073709551616 (2**64) is represented
   as 0b110_00010 (major type 6, tag 2), followed by 0b010_01001 (major



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   type 2, length 9), followed by 0x010000000000000000 (one byte 0x01
   and eight bytes 0x00).  In hexadecimal:

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


2.4.3.  Decimal Fractions and Bigfloats

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

   Bigfloats combine an integer mantissa with a base-2 scaling factor.
   They are binary floating point values that can exceed the range or
   the precision of the three IEEE 754 formats supported by CBOR
   (Section 2.3).  Bigfloats may also be used by constrained
   applications that need some basic binary floating point capability
   without the need for supporting IEEE 754.

   A decimal fraction or a bigfloat is represented as a tagged array
   that contains exactly two integer numbers: an exponent e and a
   mantissa m.  Decimal fractions (tag 4) use base-10 exponents, the
   value of a decimal fraction data item is m*(10**e).  Bigfloats (tag
   5) use base-2 exponents, the value of a bigfloat data item is
   m*(2**e).  The exponent e MUST be represented in an integer of major
   type 0 or 1, while the mantissa also can be a bignum (Section 2.4.2).

   An example of a decimal fraction is that the number 273.15 could be
   represented as 0b110_00100 (major type of 6 for the tag, additional
   information of 4 for the type of tag), followed by 0b100_00010 (major
   type of 4 for the array, additional information of 2 for the length
   of the array), followed by 0b001_00001 (major type of 1 for the first
   integer, additional information of 1 for the value of -2), followed
   by 0b000_11001 (major type of 0 for the second integer, additional
   information of 25 for a two-byte value), followed by
   0b0110101010110011 (27315 in two bytes).  In hexadecimal:

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






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

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


   Decimal fractions and bigfloats provide no representation of
   Infinity, -Infinity, or NaN; if these are needed in place of a
   decimal fraction or bigfloat, the IEEE 754 half precision
   representations from Section 2.3 can be used.  For constrained
   applications, where there is a choice between representing a specific
   number as an integer and as a decimal fraction or bigfloat (such as
   when the exponent is small and non-negative), there is a quality of
   implementation expectation that the integer representation is used
   directly.

2.4.4.  Content Hints

   The tags in this section are for content hints that might be used by
   generic CBOR processors.

2.4.4.1.  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 parsed.  Tag 24 (CBOR data item) can be used to tag the
   embedded byte string as a data item encoded in CBOR format.

2.4.4.2.  Expected Later Encoding for CBOR to JSON Converters













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   Tags 21 to 23 indicate that a byte string might require a specific
   encoding when interoperating with a text-based representation.  These
   tags are useful when an encoder knows that the byte string data it is
   writing is likely to be later converted to a particular JSON-based
   usage.  That usage specifies that some strings are encoded as Base64,
   Base64url, and so on.  The encoder uses byte strings instead of doing
   the encoding itself to reduce the message size, to reduce the code
   size of the encoder, or both.  The encoder does not know whether or
   not the converter will be generic, and therefore wants to say what it
   believes is the proper way to convert binary strings to JSON.

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

   These three tag types suggest conversions to three of the base data
   encodings defined in [RFC4648].  For base64url encoding, padding is
   not used (see section 3.2 of RFC 4648), that is all trailing equals
   signs ("=") are removed from the base64url encoded string.  Later
   tags might be defined for other data encodings of RFC 4648, or of
   other ways to encode binary data in strings.

2.4.4.3.  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.

   o  Tag 32 is for URIs, as defined in [RFC3986];

   o  Tags 33 and 34 are for base64url and base64 encoded text strings,
      as defined in [RFC4648];

   o  Tag 35 is for regular expressions in PCRE/JavaScript syntax
      [ECMA262].

   o  Tag 36 is for MIME messages (including all headers), as defined in
      [RFC2045];

   Note that tag 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 in the latter case.

2.4.5.  Self-describe CBOR





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   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 and disambiguating metadata is not in use.  Here, it
   may help to have some distinguishing characteristics for the data
   itself.

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

   The serialization of this tag is 0xd9d9f7, which appears not to be in
   use as a distinguishing mark for frequently used file types.  In
   particular, it is not a valid start of a Unicode text in any Unicode
   encoding if followed by a valid CBOR data item.

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

3.  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, the second value is an integer,
   followed by zero or more floating point numbers" or "a map whose keys
   are byte strings that has to contain at least one pair whose key is
   0xab01".

   This specification puts no restrictions on CBOR-based protocols.  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.




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   This section discusses some considerations in creating CBOR-based
   protocols.  It is advisory only, and explicitly excludes any language
   from RFC 2119 other than words that could be interpreted as "MAY" in
   the RFC 2119 sense.

3.1.  CBOR in Streaming Applications

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

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

   Note that some applications and protocols will not want to use
   indefinite length encoding.  Using indefinite length encoding allows
   an encoder to not need to marshall 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.

3.2.  Generic Encoders and Decoders

   A generic CBOR decoder can decode all well-formed CBOR data and
   present them to an application.  CBOR data are well-formed if the
   structure of the initial bytes and the byte strings/data items
   implied by their values is followed and no extraneous data follows
   (Appendix C).

   Even though CBOR attempts to minimize these cases, not all well-
   formed CBOR data are valid: for example, the format excludes simple
   values below 32 that are encoded with an extension byte.  Also,
   specific tags may make semantic constraints that may be violated,
   such as by including a tag in a bignum tag or by following a byte
   string within a date tag.  Finally, the data may be invalid, such as
   invalid UTF-8 strings or date strings that do not conform to
   [RFC3339].  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
   code points had not been registered at the time the encoder/decoder
   has been written (Section 3.5).



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   Generic decoders provide ways to present well-formed CBOR values,
   both valid and invalid, to an application.  The diagnostic notation
   (Section 6) may be used to present well-formed CBOR values to humans.

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

3.3.  Syntax Errors

   A decoder encountering a CBOR data item that is not well-formed
   generally can choose to completely fail the decoding (issue an error
   and/or stop processing altogether), substitute the problematic data
   and data items using an decoder-specific convention that clearly
   indicates there has been a problem, or it might take some other
   action.

3.3.1.  Incomplete CBOR data items

   The representation of a CBOR data item has a specific length,
   determined by its initial bytes and by the structure of any data
   items enclosed in the data items.  If less data is available, this
   can be treated as a syntax error.  A decoder may also implement
   incremental parsing, that is, decode the data item as far as it is
   available and present the data found so far, (such as in an event-
   based interface) with the option of continuing the decoding once
   further data are available.

   Examples of incomplete data items include:

   o  a decoder expecting a certain number of array or map entries
      instead encounters the end of the data

   o  a decoder processing what it expects to be the last pair in a map
      comes to the end of the data

   o  a decoder has just seen a tag and then encounters the end of the
      data

   o  a decoder has seen the beginning of an indefinite length item but
      encounters the end of the data before it sees the "break" stop
      code









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3.3.2.  Malformed Indefinite Length Items

   Examples of malformed indefinite length data items include:

   o  within an indefinite length byte string or text, a decoder finds
      an item that is not of the appropriate major type before it finds
      the "break" stop code,

   o  within an indefinite length map, a decoder encounters the "break"
      stop code immediately after reading a key (the value is missing).

3.3.3.  Unknown Additional Information Values

   At the time this document is written, some additional information
   values are unassigned and reserved for future versions of this
   document (see Section 5.2).  Since the overall syntax for these
   additional information values is not yet defined, a decoder that sees
   an additional information value that it does not understand cannot
   continue parsing.

3.4.  Other Decoding Errors

   A CBOR data item may be syntactically well-formed, but present a
   problem with interpreting the data encoded in it in the CBOR data
   model.  Generally speaking, a decoder that finds a data item with
   such a problem might issue a warning, might stop processing
   altogether, might handle the error and make the problematic value
   available to the application as such, or take some other type of
   action.

   Such problems might include:

   Duplicate keys in a map:  Generic decoders (Section 3.2) make data
      available to applications using the native CBOR data model.  That
      data model includes maps (key-value mappings with unique keys),
      not multimaps (key-value mappings where multiple entries can have
      the same key).  Thus, a generic decoder that gets a CBOR map item
      that has duplicate keys will decode to a map with only one
      instance of that key, or it might stop processing altogether.  On
      the other hand, a "streaming decoder" may not even be able to
      notice (Section 3.7).

   Inadmissible type on the value following a tag:  Tags (Section 2.4)
      specify what type of data item is supposed to follow the tag; for
      example, the tags for positive or negative bignums are supposed to
      be put on byte strings.  A decoder that decodes the tagged data
      item into a native representation (a native big integer in this
      example) is expected to check the type of the enclosed data item.



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      Even decoders that don't have such native representations
      available in their environment may perform the check on those tags
      known to them and react appropriately.

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

3.5.  Handling Unknown Simple Values and Tags

   A decoder that comes across a simple value (Section 2.3) that it does
   not recognize, such as a value that was added to the IANA registry
   after the decoder was deployed or a value that the decoder chose not
   to implement, might issue a warning, might stop processing
   altogether, might handle the error by making the unknown value
   available to the application as such (as is expected of generic
   decoders), or take some other type of action.

   A decoder that comes across a tag (Section 2.4) that it does not
   recognize, such as a tag that was added to the IANA registry after
   the decoder was deployed or a tag that the decoder chose not to
   implement, might issue a warning, might stop processing altogether,
   might handle the error and present the unknown tag value together
   with the contained data item to the application (as is expected of
   generic decoders), might ignore the tag and simply present the
   contained data item only to the application, or take some other type
   of action.

3.6.  Numbers

   For the purposes of this specification, all number representations
   for the same numeric value are equivalent.  This means that an
   encoder can encode a floating point value of 0.0 as the integer 0.
   It, however, also means that an application that expects to find
   integer values only might find floating point values if the encoder
   decides these are desirable, such as when the floating point value is
   more compact than a 64-bit integer.

   An application or protocol that uses CBOR might restrict the
   representations of numbers.  For instance, a protocol that only deals
   with integers might say that floating point numbers may not be used
   and that decoders of that protocol do not need to be able to handle
   floating point numbers.  Similarly, a protocol or application that
   uses CBOR might say that decoders need to be able to handle either
   type of number.

   CBOR-based protocols should take into account that different language
   environments pose different restrictions on the range and precision



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

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

   A CBOR-based protocol designed for compactness may want to exclude
   specific integer encodings that are longer than necessary for the
   application, such as to save the need to implement 64-bit integers.
   There is an expectation that encoders will use the most compact
   integer representation that can represent a given value.  However, a
   compact application should accept values that use a longer-than
   needed encoding (such as encoding "0" as 0b000_11101 followed by two
   bytes of 0x00) as long as the application can decode an integer of
   the given size.

3.7.  Specifying Keys for Maps

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

   If multiple types of keys are to 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
   objects, a key of integer 1 cannot be distinguished from a key of
   string "1".  This means that, if integer keys are used, the
   simultaneous use of string keys that look like numbers needs to be
   avoided.  Again, this leads to the conclusion that keys should be of
   a single CBOR type.

   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")



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   may want to reduce its overhead significantly by relying on its data
   source to maintain uniqueness.

   A CBOR-based protocol should make an intentional decision about what
   to do when a receiving application does see multiple identical keys
   in a map.  The resulting rule in the protocol should respect the CBOR
   data model: it cannot prescribe a specific handling of the entries
   with the identical keys, except that it might have a rule that having
   identical keys in a map indicates a malformed map and that the
   decoder has to stop with an error.  Duplicate keys are also
   prohibited by CBOR decoders that are using Section 3.10.

   The CBOR data model for maps does not allow ascribing semantics to
   the order of the key/value pairs in the map representation.
   Thus, it would be a very bad practice to define a CBOR-based protocol
   in such a way that changing the key/value pair order in a map would
   change the semantics, apart from trivial aspects (cache usage etc.).
   (A CBOR-based protocol can prescribe a specific order of
   serialization, such as for canonicalization.)

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

3.8.  Undefined Values

   In some CBOR-based protocols, the simple value (Section 2.3) 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.

3.9.  Canonical CBOR

   Some protocols may want encoders to only emit CBOR in a particular
   canonical format; those protocols might also have the decoders check
   that their input is canonical.  Those protocols are free to define
   what they mean by a canonical format and what encoders and decoders
   are expected to do.  This section lists some suggestions for such
   protocols.

   If a protocol considers "canonical" to mean that two encoder
   implementations starting with the same input data will produce the
   same CBOR output, the following four rules would suffice:

   o  Integers must be as small as possible.




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

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

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

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

   o  The expression of lengths in major types 2 through 5 must be as
      short as possible.  The rules for these lengths follow the above
      rule for integers.

   o  The keys in every map must be sorted lowest value to highest.
      Sorting is performed on the bytes of the representation of the key
      data items without paying attention to the 3/5 bit splitting for
      major types.  (Note that this rule allows maps that have keys of
      different types, even though that is probably a bad practice that
      could lead to errors in some canonicalization implementations.)
      The sorting rules are:

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

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

   o  Indefinite length items must be made into definite length items.

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

   CBOR tags present additional considerations for the canonicalization.
   The absence or presence of tags in a canonical format is determined
   by the optionality of the tags in the protocol.  In a CBOR-based
   protocol that allows optional tagging anywhere, the canonical format
   must not allow them.  In a protocol that requires tags in certain
   places, the tag needs to appear in the canonical format.  A CBOR-



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   based protocol that uses canonicalization might instead say that all
   tags that appear in a message must be retained regardless of whether
   they are optional.

3.10.  Strict Mode

   Some areas of application of CBOR do not require canonicalization
   (Section 3.9), but may require that different decoders reach the same
   (semantically equivalent) results, even in the presence of
   potentially malicious data.  This can be required if one application
   (such as a firewall or other protecting entity) makes a decision
   based on the data that another application, which independently
   decodes the data, relies on.

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

   A decoder in strict mode will reliably reject any data that could be
   interpreted by other decoders in different ways.  It will reliably
   reject data items with syntax errors (Section 3.3).  It will also
   expend the effort to reliably detect other decoding errors
   Section 3.4.  In particular, a strict decoder needs to have an API
   that reports an error (and does not return data) for a CBOR data item
   that contains any of the following:

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

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

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

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

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

   o  It can emit the unknown item (type, value, and, for tags, the
      decoded tagged data item) to the application calling the decoder



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      with an indication that the decoder did not recognize that tag or
      simple value.

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

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

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

4.  Converting Data Between CBOR and JSON

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

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






















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4.1.  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
   suggestion deals with these by converting them to a single substitute
   value, such as a JSON null.

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

   o  A byte string (major type 2) that is not embedded in a tag that
      specifies a proposed encoding is encoded in Base64url without
      padding and becomes a JSON string.

   o  A UTF-8 string (major type 3) becomes a JSON string.  Note that
      JSON requires escaping certain characters (RFC 4627, section 2.5):
      quotation mark (U+0022), reverse solidus (U+005C), and the "C0
      control characters" (U+0000 through U+001F).  All other characters
      are copied unchanged into the JSON UTF-8 string.

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

   o  A map (major type 5) becomes a JSON object.  This is possible
      directly only if all keys are UTF-8 strings.  A converter might
      also convert other keys into UTF-8 strings (such as by converting
      integers into strings containing their decimal representation);
      however, doing so introduces a danger of key collision.

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

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

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

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

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

   o  A bignum (major type 6, tag value 2 or 3) is represented by
      encoding its byte string in Base64url without padding and becomes



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      a JSON string.  For tag value 3 (negative bignum), a "~" (ASCII
      tilde) is inserted before the base-encoded value.  (The conversion
      to a binary blob instead of a number is to prevent a likely
      numeric overflow for the JSON decoder.)

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

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

   o  Indefinite length items are made definite before conversion.

4.2.  Converting From JSON to CBOR

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

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

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

   CBOR has been designed to generally provide a more compact encoding
   than JSON.  One implementation strategy that might come to mind is to
   perform a JSON to CBOR encoding in place in a single buffer.  This
   strategy would need to carefully consider a number of pathological
   case, such as that some strings represented with no or very few
   escapes and longer (or much longer) than 255 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



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   representations (1.1, 1e9) in JSON.  This may be hard to get right
   and any ensuing vulnerabilities may be exploited by an attacker.

5.  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.

5.1.  Extension Points

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

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

   CBOR has three major extension points:

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




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   o  the "tag" space (values in major type 6).  Again, only a small
      part of the code point space has been allocated, and the space is
      abundant (although the early numbers are more efficient than the
      later ones).  Implementations receiving an unknown tag can choose
      to simply ignore it, or to process it as an unknown tag wrapping
      the following data item.  An IANA registry is appropriate here.

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

5.2.  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 code
   point 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
   code point space by an IANA registry.  Instead, allocations out of
   this space can only be done by updating this specification.

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

6.  Diagnostic Notation

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

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

   The diagnostic notation is loosely based on JSON as it is defined in
   RFC 4627, extending it where needed.



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

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

   or the equivalent relative time as

      1(1363896240)

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

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

6.1.  Encoding indicators

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

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

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




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

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

7.  IANA Considerations

   IANA will create two registries for new CBOR values.  The registries
   will follow the rules in [RFC5226].  IANA will also assign a new MIME
   media type and an associated CoAP Content-Format entry.

7.1.  Simple Values Registry

   A registry called "CBOR Simple Values" will be created.  The initial
   values are shown in Table 2.

   New entries in the range 0 to 19 will be 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
   any contiguous block.

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

7.2.  Tags Registry

   A registry called "CBOR Tags" will be created.  The initial values
   are shown in Table 3.

   New entries in the range 0 to 23 will be assigned by Standards
   Action.  New entries in the range 24 to 255 will be assigned by
   Specification Required.  New entries in the range 256 to
   18446744073709551615 will be assigned by First Come First Served.
   The template for First Come First Served will include point of
   contact and an optional field for URL to a description of the
   semantics of the tag; the latter can be something like an Internet-
   Draft or a web page.

7.3.  Media Type ("MIME Type")

   The Internet media type [RFC6838] for CBOR data is application/cbor.



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   Type name: application

   Subtype name: cbor

   Required parameters: n/a

   Optional parameters: n/a

   Encoding considerations:  binary

   Security considerations:  See Section 8 of this document

   Interoperability considerations: n/a

   Published specification: This document

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

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

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

   Intended usage: COMMON

   Restrictions on usage: none

   Author:
     Carsten Bormann <cabo@tzi.org>

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


7.4.  CoAP Content-Format

   Media Type: application/cbor

   Encoding: -

   Id: 60

   Reference: [RFC-THIS-SPEC]



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7.5.  The +cbor Structured Syntax Suffix Registration

   Name: Concise Binary Object Representation (CBOR)

   +suffix: +cbor

   References: [RFC-THIS-SPEC]

   Encoding considerations: CBOR is a binary format.

   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".


   Interoperability considerations: n/a

   Security considerations:  See Section 8 of this document

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

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


8.  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.



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   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.

   Resource exhaustion attacks might attempt to lure a decoder into
   allocating very big data items (strings, arrays, maps) or exhaust the
   stack depth by setting up deeply nested items.  Decoders need to have
   appropriate resource management to mitigate these attacks.  (Items
   for which very large sizes are given can also attempt to exploit
   integer overflow vulnerabilities.)

   Applications where CBOR data items are examined by a gatekeeper
   function and later used by a different application may exhibit
   vulnerabilities when multiple interpretations of the data item are
   possible.  For example, an attacker could make use of duplicate keys
   in maps and precision issues in numbers to make the gatekeeper base
   its decisions on a different interpretation than the one that will be
   used by the second application.  Protocols that are used in a
   security context should be defined in such a way that these multiple
   interpretations are reliably reduced to a single one.  To facilitate
   this, encoder and decoder implementations used in such contexts
   should provide at least one strict mode of operation (Section 3.10).

9.  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 recent 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, Joe Hildebrand, Keith Moore, Matthew
   Lepinski, Nico Williams, Phillip Hallam-Baker, Ray Polk, Tim Bray,
   Tony Finch, Tony Hansen, and Yaron Sheffer.




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10.  References

10.1.  Normative References

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

10.2.  Informative References

   [ASN.1]    International Telecommunications 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", 2013, <http://bsonspec.org/>.

   [ECMA262]  European Computer Manufacturers Association, "ECMAScript
              Language Specification 5.1 Edition", ECMA Standard
              ECMA-262, June 2011, <http://www.ecma-international.org/
              publications/files/ecma-st/ECMA-262.pdf>.

   [I-D.ietf-lwig-terminology]
              Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained Node Networks", draft-ietf-lwig-terminology-05
              (work in progress), July 2013.

   [MessagePack]
              FURUHASHI Sadayuki, "MessagePack", 2013,
              <http://msgpack.org/>.

   [RFC0713]  Haverty, J., "MSDTP-Message Services Data Transmission
              Protocol", RFC 713, April 1976.

   [RFC2045]  Freed, N. and N.S. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.




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   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, December 2005.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

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

   [UBJSON]   The Buzz Media, "Universal Binary JSON Specification",
              2013, <http://ubjson.org/>.

   [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

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



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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                   |
   |                      |                                            |
   | -1844674407370955161 | 0x3bffffffffffffffff                       |
   | 6                    |                                            |
   |                      |                                            |
   | -1844674407370955161 | 0xc349010000000000000000                   |
   | 7                    |                                            |
   |                      |                                            |
   | -1                   | 0x20                                       |
   |                      |                                            |
   | -10                  | 0x29                                       |
   |                      |                                            |
   | -100                 | 0x3863                                     |
   |                      |                                            |
   | -1000                | 0x3903e7                                   |
   |                      |                                            |
   | 0.0                  | 0xf90000                                   |
   |                      |                                            |
   | -0.0                 | 0xf98000                                   |
   |                      |                                            |
   | 1.0                  | 0xf93c00                                   |
   |                      |                                            |
   | 1.1                  | 0xfb3ff199999999999a                       |



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   |                      |                                            |
   | 1.5                  | 0xf93e00                                   |
   |                      |                                            |
   | 65504.0              | 0xf97bff                                   |
   |                      |                                            |
   | 100000.0             | 0xfa47c35000                               |
   |                      |                                            |
   | 3.4028234663852886e+ | 0xfa7f7fffff                               |
   | 38                   |                                            |
   |                      |                                            |
   | 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                                       |
   |                      |                                            |



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   | simple(24)           | 0xf818                                     |
   |                      |                                            |
   | simple(255)          | 0xf8ff                                     |
   |                      |                                            |
   | 0("2013-03-21T20:04: | 0xc074323031332d30332d32315432303a30343a30 |
   | 00Z")                | 305a                                       |
   |                      |                                            |
   | 1(1363896240)        | 0xc11a514b67b0                             |
   |                      |                                            |
   | 1(1363896240.5)      | 0xc1fb41d452d9ec200000                     |
   |                      |                                            |
   | 23(h'01020304')      | 0xd74401020304                             |
   |                      |                                            |
   | 24(h'6449455446')    | 0xd818456449455446                         |
   |                      |                                            |
   | 32("http://www.examp | 0xd82076687474703a2f2f7777772e6578616d706c |
   | le.com")             | 652e636f6d                                 |
   |                      |                                            |
   | 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,   | 0x98190102030405060708090a0b0c0d0e0f101112 |
   | 7, 8, 9, 10, 11, 12, | 131415161718181819                         |
   | 13, 14, 15, 16, 17,  |                                            |
   | 18, 19, 20, 21, 22,  |                                            |
   | 23, 24, 25]          |                                            |
   |                      |                                            |



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   | {}                   | 0xa0                                       |
   |                      |                                            |
   | {1: 2, 3: 4}         | 0xa201020304                               |
   |                      |                                            |
   | {"a": 1, "b": [2,    | 0xa26161016162820203                       |
   | 3]}                  |                                            |
   |                      |                                            |
   | ["a", {"b": "c"}]    | 0x826161a161626163                         |
   |                      |                                            |
   | {"a": "A", "b": "B", | 0xa561616141616261426163614361646144616561 |
   | "c": "C", "d": "D",  | 45                                         |
   | "e": "E"}            |                                            |
   |                      |                                            |
   | (_ h'0102',          | 0x5f42010243030405ff                       |
   | h'030405')           |                                            |
   |                      |                                            |
   | (_ "strea", "ming")  | 0x7f657374726561646d696e67ff               |
   |                      |                                            |
   | [_ ]                 | 0x9fff                                     |
   |                      |                                            |
   | [_ 1, [2, 3], [_ 4,  | 0x9f018202039f0405ffff                     |
   | 5]]                  |                                            |
   |                      |                                            |
   | [_ 1, [2, 3], [4,    | 0x9f01820203820405ff                       |
   | 5]]                  |                                            |
   |                      |                                            |
   | [1, [2, 3], [_ 4,    | 0x83018202039f0405ff                       |
   | 5]]                  |                                            |
   |                      |                                            |
   | [1, [_ 2, 3], [4,    | 0x83019f0203ff820405                       |
   | 5]]                  |                                            |
   |                      |                                            |
   | [_ 1, 2, 3, 4, 5, 6, | 0x9f0102030405060708090a0b0c0d0e0f10111213 |
   | 7, 8, 9, 10, 11, 12, | 1415161718181819ff                         |
   | 13, 14, 15, 16, 17,  |                                            |
   | 18, 19, 20, 21, 22,  |                                            |
   | 23, 24, 25]          |                                            |
   |                      |                                            |
   | {_ "a": 1, "b": [_   | 0xbf61610161629f0203ffff                   |
   | 2, 3]}               |                                            |
   |                      |                                            |
   | ["a", {_ "b": "c"}]  | 0x826161bf61626163ff                       |
   |                      |                                            |
   | {_ "Fun": true,      | 0xbf6346756ef563416d7421ff                 |
   | "Amt": -2}           |                                            |
   +----------------------+--------------------------------------------+





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Appendix B.  Jump Table

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

   +-----------------+-------------------------------------------------+
   | Byte            | Structure/Semantics                             |
   +-----------------+-------------------------------------------------+
   | 0x00..0x17      | Integer 0x00..0x17 (0..23)                      |
   |                 |                                                 |
   | 0x18            | Unsigned integer (one-byte uint8_t follows)     |
   |                 |                                                 |
   | 0x19            | Unsigned integer (two-byte uint16_t follows)    |
   |                 |                                                 |
   | 0x1a            | Unsigned integer (four-byte uint32_t follows)   |
   |                 |                                                 |
   | 0x1b            | Unsigned integer (eight-byte uint64_t follows)  |
   |                 |                                                 |
   | 0x20..0x37      | Negative Integer -1-0x00..-1-0x17 (-1..-24)     |
   |                 |                                                 |
   | 0x38            | Negative Integer -1-n (one-byte uint8_t for n   |
   |                 | follows)                                        |
   |                 |                                                 |
   | 0x39            | Negative integer -1-n (two-byte uint16_t for n  |
   |                 | follows)                                        |
   |                 |                                                 |
   | 0x3a            | Negative integer -1-n (four-byte uint32_t for n |
   |                 | follows)                                        |
   |                 |                                                 |
   | 0x3b            | Negative integer -1-n (eight-byte uint64_t for  |
   |                 | n follows)                                      |
   |                 |                                                 |
   | 0x40..0x57      | byte string (0x00..0x17 bytes follow)           |
   |                 |                                                 |
   | 0x58            | byte string (one-byte uint8_t for n, and then n |
   |                 | bytes follow)                                   |
   |                 |                                                 |
   | 0x59            | byte string (two-byte uint16_t for n, and then  |
   |                 | n bytes follow)                                 |
   |                 |                                                 |
   | 0x5a            | byte string (four-byte uint32_t for n, and then |
   |                 | n bytes follow)                                 |
   |                 |                                                 |
   | 0x5b            | byte string (eight-byte uint64_t for n, and     |
   |                 | then n bytes follow)                            |
   |                 |                                                 |



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



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   |                 | 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 2.4.1)                                  |
   |                 |                                                 |
   | 0xc1            | Epoch-based date/time (data item follows, see   |
   |                 | Section 2.4.1)                                  |
   |                 |                                                 |
   | 0xc2            | Positive bignum (data item "byte string"        |
   |                 | follows)                                        |
   |                 |                                                 |
   | 0xc3            | Negative bignum (data item "byte string"        |
   |                 | follows)                                        |
   |                 |                                                 |
   | 0xc4            | Decimal Fraction (data item "array" follows,    |
   |                 | see Section 2.4.3)                              |
   |                 |                                                 |
   | 0xc5            | Bigfloat (data item "array" follows, see        |
   |                 | Section 2.4.3)                                  |
   |                 |                                                 |
   | 0xc6..0xd4      | (tagged item, tag to be assigned by IANA)       |
   |                 |                                                 |
   | 0xd5..0xd7      | Expected Conversion (data item follows, see     |
   |                 | Section 2.4.4.2)                                |
   |                 |                                                 |
   | 0xd8..0xdb      | (more tagged items, 1/2/4/8 bytes and then a    |
   |                 | data item follow)                               |
   |                 |                                                 |
   | 0xe0..0xf3      | (simple value to be assigned by IANA)           |
   |                 |                                                 |
   | 0xf4            | False                                           |
   |                 |                                                 |
   | 0xf5            | True                                            |
   |                 |                                                 |
   | 0xf6            | Null                                            |
   |                 |                                                 |
   | 0xf7            | Undefined                                       |
   |                 |                                                 |
   | 0xf8            | (simple value to be assigned by IANA, one byte  |
   |                 | follows)                                        |
   |                 |                                                 |
   | 0xf9            | Half-Precision Float (two-byte IEEE 754)        |
   |                 |                                                 |
   | 0xfa            | Single-Precision Float (four-byte IEEE 754)     |
   |                 |                                                 |



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

                   Table 4: Jump Table for Initial Byte

Appendix C.  Pseudocode

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

   o  the pseudo-code does not "fail";

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

   The pseudo-code has the following prerequisites:

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

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

   o  Arithmetic works as in C.

   o  All variables are unsigned integers of sufficient range.

   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;



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       case 5: for (i = 0; i < val*2; i++) well_formed(); break;
       case 6: well_formed(); break;     // 1 embedded data item
     }
     return mt;                    // finite data item
   }

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

              Figure 1: Pseudo-Code for well-formedness check

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

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

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



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       *p++ = mt + 24;
       *p++ = ui;
     } else
          ...

            Figure 2: Pseudo-code for encoding a signed integer

Appendix D.  Half-precision

   As half-precision floating point numbers were only added to IEEE 754
   in 2008, 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).

   #include <math.h>

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

               Figure 3: C code for a half-precision decoder

   import struct
   from math import ldexp

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

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

            Figure 4: Python code for a half-precision decoder




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

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

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

   Note that the discussion here is not meant to be a criticism of any
   format: to the best of our knowledge, no format before CBOR was meant
   to cover CBOR's objectives in the priority we have assigned them.  A
   brief recap of the objectives from Section 1.1 is:

   1.  unambiguously encode common data formats from Internet standards

   2.  code compactness for encoder or decoder

   3.  no schema description needed

   4.  reasonably compact serialization

   5.  applicable to constrained and unconstrained applications

   6.  good JSON conversion

   7.  extensibility

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

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









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

E.2.  MessagePack

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

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

E.3.  BSON

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

E.4.  UBJSON

   [UBJSON] has a design goal to make JSON faster and somewhat smaller,
   using a binary format that is limited to exactly the data model JSON
   uses.  Thus, there is expressly no intention to support, for example,
   binary data; however, there is a "high-precision number", expressed
   as a character string in JSON syntax.  UBJSON is not optimized for
   code compactness, and its type byte coding is optimized for human
   recognition and not for compact representation of native types such
   as small integers.  Although UBJSON is mostly counted, it provides a



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   reserved "unknown-length" value to support streaming of arrays and
   maps (JSON objects).  Within these containers, UBJSON also has a
   "Noop" type for padding.

E.5.  MSDTP: RFC 713

   A very early example of a compact message format is described in
   [RFC0713], defined in 1976.  It is included here for its historical
   value, not because it was ever widely used.

E.6.  Conciseness On The Wire

   While CBOR's design objective of code compactness for encoders and
   decoders is higher than its objective of conciseness on the wire,
   many people focus on the wire size.  Table 5 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.

   (Entries marked with an asterisk have not been checked against an
   implementation and might be applying some liberty in translating the
   CBOR data model to that format.  Corrections are appreciated.)

   +---------------+-------------------------+-------------------------+
   | Format        | [1, [2, 3]]             | [_ 1, [2, 3]]           |
   +---------------+-------------------------+-------------------------+
   | RFC 713*      | c2 05 81 c2 02 82 83    |                         |
   |               |                         |                         |
   | ASN.1 BER*    | 30 0b 02 01 01 30 06 02 | 30 80 02 01 01 30 06 02 |
   |               | 01 02 02 01 03          | 01 02 02 01 03 00 00    |
   |               |                         |                         |
   | MessagePack   | 92 01 92 02 03          |                         |
   |               |                         |                         |
   | BSON          | 22 00 00 00 10 30 00 01 |                         |
   |               | 00 00 00 04 31 00 13 00 |                         |
   |               | 00 00 10 30 00 02 00 00 |                         |
   |               | 00 10 31 00 03 00 00 00 |                         |
   |               | 00 00                   |                         |
   |               |                         |                         |
   | UBJSON        | 61 02 42 01 61 02 42 02 | 61 ff 42 01 61 02 42 02 |
   |               | 42 03                   | 42 03 45*               |
   |               |                         |                         |
   | CBOR          | 82 01 82 02 03          | 9f 01 82 02 03 ff       |
   +---------------+-------------------------+-------------------------+

           Table 5: Examples for different levels of conciseness





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Authors' Addresses

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

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


   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org


































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