rfc7049.xml

<?xml version='1.0' encoding='utf-8'?>
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
<!ENTITY RFC2045 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2045.xml">
<!ENTITY RFC2119 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY RFC3339 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3339.xml">
<!ENTITY RFC3629 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3629.xml">
<!ENTITY RFC3986 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3986.xml">
<!ENTITY RFC4287 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4287.xml">
<!ENTITY RFC4648 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4648.xml">
<!ENTITY RFC5226 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5226.xml">
<!ENTITY RFC0713 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.0713.xml">
<!ENTITY RFC4627 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4627.xml">
<!ENTITY RFC6838 SYSTEM "https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6838.xml">
]>
<rfc submissionType="IETF" number="7049" category="std" consensus="yes">
	<!-- Generated by id2xml 1.4.4 on 2019-05-13T05:48:44Z -->
	<?rfc compact="yes"?>
	<?rfc text-list-symbols="o*+-"?>
	<?rfc subcompact="no"?>
	<?rfc sortrefs="yes"?>
	<?rfc symrefs="yes"?>
	<?rfc strict="yes"?>
	<?rfc toc="yes"?>
	<front>
	<title abbrev="CBOR">Concise Binary Object Representation (CBOR)</title>
	<author fullname="Carsten Bormann" initials="C." surname="Bormann">
	<organization>Universitaet Bremen TZI</organization>
	<address><postal><street>Postfach 330440</street>
	<street>D-28359 Bremen</street>
	<street>Germany</street>
	</postal>
	<phone>+49-421-218-63921</phone>
	<email>cabo@tzi.org</email>
	</address>
	</author>

	<author fullname="Paul Hoffman" initials="P." surname="Hoffman">
	<organization>VPN Consortium</organization>
	<address><email>paul.hoffman@vpnc.org</email>
	</address>
	</author>

	<date month="October" year="2013"/>
	<abstract><t>
   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.</t>

	</abstract>
	</front>

	<middle>
	<section title="Introduction" anchor="section-1"><t>
   There are hundreds of standardized formats for binary representation
   of structured data (also known as binary serialization formats).  Of
   those, some are for specific domains of information, while others are
   generalized for arbitrary data.  In the IETF, probably the best-known
   formats in the latter category are ASN.1's BER and DER <xref target="ASN.1"/>.</t>

	<t>
   The format defined here follows some specific design goals that are
   not well met by current formats.  The underlying data model is an
   extended version of the JSON data model <xref target="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 that starts
   from JSON.</t>

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

	<section title="Objectives" anchor="section-1.1"><t>
   The objectives of CBOR, roughly in decreasing order of importance,
   are:</t>

	<t><list style="numbers"><t>The representation must be able to unambiguously encode most
       common data formats used in Internet standards.<list style="symbols"><t>It must represent a reasonable set of basic data types and
          structures using binary encoding.  "Reasonable" here is
          largely influenced by the capabilities of JSON, with the major
          addition of binary byte strings.  The structures supported are
          limited to arrays and trees; loops and lattice-style graphs
          are not supported.</t>

	<t>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.</t>

	</list>
	</t>

	<t>The code for an encoder or decoder must be able to be compact in
       order to support systems with very limited memory, processor
       power, and instruction sets.<list style="symbols"><t>An encoder and a decoder need to be implementable in a very
          small amount of code (for example, in class 1 constrained
          nodes as defined in <xref target="CNN-TERMS"/>).</t>

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

	</list>
	</t>

	<t>Data must be able to be decoded without a schema description.<list style="symbols"><t>Similar to JSON, encoded data should be self-describing so
          that a generic decoder can be written.</t>

	</list>
	</t>

	<t>The serialization must be reasonably compact, but data
       compactness is secondary to code compactness for the encoder and
       decoder.<list style="symbols"><t>"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.</t>

	</list>
	</t>

	<t>The format must be applicable to both constrained nodes and high-
       volume applications.<list style="symbols"><t>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.</t>

	</list>
	</t>

	<t>The format must support all JSON data types for conversion to and
       from JSON.<list style="symbols"><t>It must support a reasonable level of conversion as long as
          the data represented is within the capabilities of JSON.  It
          must be possible to define a unidirectional mapping towards
          JSON for all types of data.</t>

	</list>
	</t>

	<t>The format must be extensible, and the extended data must be
       decodable by earlier decoders.<list style="symbols"><t>The format is designed for decades of use.</t>

	<t>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.</t>

	<t>The format must be able to be extended in the future by later
          IETF standards.</t>

	</list>
	</t>

	</list>
	</t>

	</section>

	<section title="Terminology" anchor="section-1.2"><t>
   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
   <xref target="RFC2119"/> and indicate requirement levels for compliant CBOR
   implementations.</t>

	<t>
   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").</t>

	<t>
   This specification makes use of the following terminology:</t>

	<t><list style="hanging" hangIndent="3"><t hangText="Data item:">
	A single piece of CBOR data.  The structure of a data
	<vspace blankLines="0"/>
	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.
	</t>

	<t hangText="Decoder:">
	A process that decodes a CBOR data item and makes it
	<vspace blankLines="0"/>
	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.
	</t>

	<t hangText="Encoder:">
	A process that generates the representation format of a
	<vspace blankLines="0"/>
	CBOR data item from application information.
	</t>

	<t hangText="Data Stream:">
	A sequence of zero or more data items, not further
	<vspace blankLines="0"/>
	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".
	</t>

	<t hangText="Well-formed:">
	A data item that follows the syntactic structure of
	<vspace blankLines="0"/>
	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.
	</t>

	<t hangText="Valid:">
	A data item that is well-formed and also follows the semantic
	<vspace blankLines="0"/>
	restrictions that apply to CBOR data items.
	</t>

	<t hangText="Stream decoder:">
	A process that decodes a data stream and makes each
	<vspace blankLines="0"/>
	of the data items in the sequence available to an application as
      they are received.
	</t>

	</list>
	</t>

	<t>
   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, it is split into three bits and five bits.</t>

	</section>

	</section>

	<section title="Specification of the CBOR Encoding" anchor="section-2"><t>
   A CBOR-encoded data item is structured and encoded as described in
   this section.  The encoding is summarized in Table 5.</t>

	<t>
   The initial byte of each data item contains both information about
   the major type (the high-order 3 bits, described in <xref target="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
   <xref target="section-2.2"/>.  Additional information values 28 to 30 are reserved for
   future expansion.</t>

	<t>
   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.</t>

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

	<section title="Major Types" anchor="section-2.1"><t>
   The following lists the major types and the additional information
   and other bytes associated with the type.</t>

	<t><list style="hanging" hangIndent="3"><t hangText="Major type 0:">
	an unsigned integer.  The 5-bit additional information
	<vspace blankLines="0"/>
	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.
	</t>

	<t hangText="Major type 1:">
	a negative integer.  The encoding follows the rules
	<vspace blankLines="0"/>
	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.
	</t>

	<t hangText="Major type 2:">
	a byte string.  The string's length in bytes is
	<vspace blankLines="0"/>
	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
	<vspace blankLines="1"/>
	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.
	</t>

	<t hangText="Major type 3:">
	a text string, specifically a string of Unicode
	<vspace blankLines="0"/>
	characters that is encoded as UTF-8 <xref target="RFC3629"/>.  The format of this
      type is identical to that of byte 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").
	</t>

	<t hangText="Major type 4:">
	an array of data items.  Arrays are also called lists,
	<vspace blankLines="0"/>
	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.
	</t>

	<t hangText="Major type 5:">
	a map of pairs of data items.  Maps are also called
	<vspace blankLines="0"/>
	tables, dictionaries, hashes, or objects (in JSON).  A map is
      comprised of pairs of data items, each pair consisting of a key
      that is immediately followed by a value.  The 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
      <xref target="section-3.7"/>.
	</t>

	<t hangText="Major type 6:">
	optional semantic tagging of other major types.  See
	<vspace blankLines="0"/>
	<xref target="section-2.4"/>.
	</t>

	<t hangText="Major type 7:">
	floating-point numbers and simple data types that need
	<vspace blankLines="0"/>
	no content, as well as the "break" stop code.  See <xref target="section-2.3"/>.
	</t>

	</list>
	</t>

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

	<t>
   In major types 6 and 7, many of the possible values are reserved for
   future specification.  See <xref target="section-7"/> for more information on these
   values.</t>

	</section>

	<section title="Indefinite Lengths for Some Major Types" anchor="section-2.2"><t>
   Four CBOR items (arrays, maps, byte strings, and text strings) can be
   encoded with an indefinite length using additional information value
   31.  This is useful if the encoding of the item needs to begin before
   the number of items inside the array or map, or the total length of
   the string, is known.  (The application of this is often referred to
   as "streaming" within a data item.)</t>

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

	<section title="Indefinite-Length Arrays and Maps" anchor="section-2.2.1"><t>
   Indefinite-length arrays and maps are simply opened without
   indicating the number of data items that will be included in the
   array or map, using the additional information value of 31.  The
   initial major type and additional information byte is followed by the
   elements of the array or map, just as they would be in other arrays
   or maps.  The end of the array or map is indicated by encoding a
   "break" stop code in a place where the next data item would normally
   have been included.  The "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 array or map.
   That is, the "break" stop code comes after the last item in the array
   or map, and it cannot occur anywhere else in place of a data item.
   In this way, indefinite-length arrays and maps look identical to
   other arrays and maps except for beginning with the additional
   information value 31 and ending with the "break" stop code.</t>

	<t>
   Arrays and maps with indefinite lengths allow any number of items
   (for arrays) and key/value pairs (for maps) to be given before the
   "break" stop code.  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.</t>

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

	<figure><artwork><![CDATA[
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
]]></artwork>
	</figure>
	<t>
   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:</t>

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

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

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

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
]]></artwork>
	</figure>
	<t>
   An example of an indefinite-length map (that happens to have two
   key/value pairs) might be:</t>

	<figure><artwork><![CDATA[
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"
]]></artwork>
	</figure>
	</section>

	<section title="Indefinite-Length Byte Strings and Text Strings" anchor="section-2.2.2"><t>
   Indefinite-length byte strings and text strings are actually a
   concatenation of zero or more definite-length byte or text strings
   ("chunks") that are together treated as one contiguous string.
   Indefinite-length strings are opened with the major type and
   additional information value of 31, but what follows are a series of
   byte or text strings that have definite lengths (the chunks).  The
   end of the series of chunks is indicated by encoding the "break" stop
   code (0b111_11111) in a place where the next chunk in the series
   would occur.  The contents of the chunks are concatenated together,
   and the overall length of the indefinite-length string will be the
   sum of the lengths of all of the chunks.  In summary, an indefinite-
   length string is encoded similarly to how an indefinite-length array
   of its chunks would be encoded, except that the major type of the
   indefinite-length string is that of a (text or byte) string and
   matches the major types of its chunks.</t>

	<t>
   For indefinite-length byte strings, every data item (chunk) between
   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.</t>

	<t>
   For example, assume the sequence:</t>

	<t>
   0b010_11111 0b010_00100 0xaabbccdd 0b010_00011 0xeeff99 0b111_11111</t>

	<figure><artwork><![CDATA[
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"
]]></artwork>
	</figure>
	<t>
   After decoding, this results in a single byte string with seven
   bytes: 0xaabbccddeeff99.</t>

	<t>
   Text strings with indefinite lengths act the same as byte strings
   with indefinite lengths, except that all their chunks MUST be
   definite-length text strings.  Note that this implies that the bytes
   of a single UTF-8 character cannot be spread between chunks: a new
   chunk can only be started at a character boundary.</t>

	</section>

	</section>

	<section title="Floating-Point Numbers and Values with No Content" anchor="section-2.3"><t>
   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.</t>

	<texttable title="Values for Additional Information in Major Type 7" anchor="ref-values-for-additional-information-in-major-type-7" style="full"><ttcol> 5-Bit Value</ttcol>
	<ttcol> Semantics</ttcol>
	<c>0..23</c>
	<c>Simple value (value 0..23)</c>
	<c>24</c>
	<c>Simple value (value 32..255 in following byte)</c>
	<c>25</c>
	<c>IEEE 754 Half-Precision Float (16 bits follow)</c>
	<c>26</c>
	<c>IEEE 754 Single-Precision Float (32 bits follow)</c>
	<c>27</c>
	<c>IEEE 754 Double-Precision Float (64 bits follow)</c>
	<c>28-30</c>
	<c>(Unassigned)</c>
	<c>31</c>
	<c>"break" stop code for indefinite-length items</c>
	</texttable>
	<t>
   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.</t>

	<texttable title="Simple Values" anchor="ref-simple-values" style="full"><ttcol> Value</ttcol>
	<ttcol> Semantics</ttcol>
	<c>0..19</c>
	<c>(Unassigned)</c>
	<c>20</c>
	<c>False</c>
	<c>21</c>
	<c>True</c>
	<c>22</c>
	<c>Null</c>
	<c>23</c>
	<c>Undefined value</c>
	<c>24..31</c>
	<c>(Reserved)</c>
	<c>32..255</c>
	<c>(Unassigned)</c>
	</texttable>
	<t>
   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.)</t>

	</section>

	<section title="Optional Tagging of Items" anchor="section-2.4"><t>
   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.</t>

	<t>
   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 (<xref target="section-2.4.2"/>).  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.</t>

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

	<t>
   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.</t>

	<t>
   IANA maintains a registry of tag values as described in <xref target="section-7.2"/>.
   Table 3 provides a list of initial values, with definitions in the
   rest of this section.</t>

	<texttable title="Values for Tags" anchor="ref-values-for-tags" style="full"><ttcol> Tag</ttcol>
	<ttcol> Data Item</ttcol>
	<ttcol> Semantics</ttcol>
	<c>0</c>
	<c>UTF-8 string</c>
	<c>Standard date/time string; see</c>
	<c></c>
	<c></c>
	<c>Section 2.4.1</c>
	<c>1</c>
	<c>multiple</c>
	<c>Epoch-based date/time; see</c>
	<c></c>
	<c></c>
	<c>Section 2.4.1</c>
	<c>2</c>
	<c>byte string</c>
	<c>Positive bignum; see Section</c>
	<c></c>
	<c></c>
	<c>2.4.2</c>
	<c>3</c>
	<c>byte string</c>
	<c>Negative bignum; see Section</c>
	<c></c>
	<c></c>
	<c>2.4.2</c>
	<c>4</c>
	<c>array</c>
	<c>Decimal fraction; see Section</c>
	<c></c>
	<c></c>
	<c>2.4.3</c>
	<c>5</c>
	<c>array</c>
	<c>Bigfloat; see Section 2.4.3</c>
	<c>6..20</c>
	<c>(Unassigned)</c>
	<c>(Unassigned)</c>
	<c>21</c>
	<c>multiple</c>
	<c>Expected conversion to</c>
	<c></c>
	<c></c>
	<c>base64url encoding; see</c>
	<c></c>
	<c></c>
	<c>Section 2.4.4.2</c>
	<c>22</c>
	<c>multiple</c>
	<c>Expected conversion to base64</c>
	<c></c>
	<c></c>
	<c>encoding; see Section 2.4.4.2</c>
	<c>23</c>
	<c>multiple</c>
	<c>Expected conversion to base16</c>
	<c></c>
	<c></c>
	<c>encoding; see Section 2.4.4.2</c>
	<c>24</c>
	<c>byte string</c>
	<c>Encoded CBOR data item; see</c>
	<c></c>
	<c></c>
	<c>Section 2.4.4.1</c>
	<c>25..31</c>
	<c>(Unassigned)</c>
	<c>(Unassigned)</c>
	<c>32</c>
	<c>UTF-8 string</c>
	<c>URI; see Section 2.4.4.3</c>
	<c>33</c>
	<c>UTF-8 string</c>
	<c>base64url; see Section 2.4.4.3</c>
	<c>34</c>
	<c>UTF-8 string</c>
	<c>base64; see Section 2.4.4.3</c>
	<c>35</c>
	<c>UTF-8 string</c>
	<c>Regular expression; see</c>
	<c></c>
	<c></c>
	<c>Section 2.4.4.3</c>
	<c>36</c>
	<c>UTF-8 string</c>
	<c>MIME message; see Section</c>
	<c></c>
	<c></c>
	<c>2.4.4.3</c>
	<c>37..55798</c>
	<c>(Unassigned)</c>
	<c>(Unassigned)</c>
	<c>55799</c>
	<c>multiple</c>
	<c>Self-describe CBOR; see</c>
	<c></c>
	<c></c>
	<c>Section 2.4.5</c>
	<c>55800+</c>
	<c>(Unassigned)</c>
	<c>(Unassigned)</c>
	</texttable>
	<section title="Date and Time" anchor="section-2.4.1"><t>
   Tag value 0 is for date/time strings that follow the standard format
   described in <xref target="RFC3339"/>, as refined by Section 3.3 of <xref target="RFC4287"/>.</t>

	<t>
   Tag value 1 is for numerical representation of seconds relative to
   1970-01-01T00:00Z in UTC time.  (For the non-negative values that the
   Portable Operating System Interface (POSIX) defines, the number of
   seconds is counted in the same way as for POSIX "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.</t>

	</section>

	<section title="Bignums" anchor="section-2.4.2"><t>
   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.</t>

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

	<t><list style="hanging" hangIndent="3"><t hangText="C2">
	-- Tag 2
	<vspace blankLines="0"/>
	29                     -- Byte string of length 9
         010000000000000000  -- Bytes content
	</t>

	</list>
	</t>

	</section>

	<section title="Decimal Fractions and Bigfloats" anchor="section-2.4.3"><t>
   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.</t>

	<t>
   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
   (<xref target="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.</t>

	<t>
   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 (<xref target="section-2.4.2"/>).</t>

	<t>
   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:</t>

	<figure><artwork><![CDATA[
C4             -- Tag 4
   82          -- Array of length 2
      21       -- -2
      19 6ab3  -- 27315
]]></artwork>
	</figure>
	<t>
   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:</t>

	<figure><artwork><![CDATA[
C5             -- Tag 5
   82          -- Array of length 2
      20       -- -1
      03       -- 3
]]></artwork>
	</figure>
	<t>
   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 <xref target="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.</t>

	</section>

	<section title="Content Hints" anchor="section-2.4.4"><t>
   The tags in this section are for content hints that might be used by
   generic CBOR processors.</t>

	<section title="Encoded CBOR Data Item" anchor="section-2.4.4.1"><t>
   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.</t>

	</section>

	<section title="Expected Later Encoding for CBOR-to-JSON Converters" anchor="section-2.4.4.2"><t>
   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.</t>

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

	<t>
   These three tag types suggest conversions to three of the base data
   encodings defined in <xref target="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 for
   other ways to encode binary data in strings.</t>

	</section>

	<section title="Encoded Text" anchor="section-2.4.4.3"><t>
   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.</t>

	<t><list style="symbols"><t>Tag 32 is for URIs, as defined in <xref target="RFC3986"/>;</t>

	<t>Tags 33 and 34 are for base64url- and base64-encoded text strings,
      as defined in <xref target="RFC4648"/>;</t>

	<t>Tag 35 is for regular expressions in Perl Compatible Regular
      Expressions (PCRE) / JavaScript syntax <xref target="ECMA262"/>.</t>

	<t>Tag 36 is for MIME messages (including all headers), as defined in
      <xref target="RFC2045"/>;</t>

	</list>
	</t>

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

	</section>

	</section>

	<section title="Self-Describe CBOR" anchor="section-2.4.5"><t>
   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.</t>

	<t>
   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.</t>

	<t>
   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.</t>

	<t>
   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.</t>

	</section>

	</section>

	</section>

	<section title="Creating CBOR-Based Protocols" anchor="section-3"><t>
   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.</t>

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

	<t>
   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.</t>

	<t>
   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 sense of RFC 2119.</t>

	<section title="CBOR in Streaming Applications" anchor="section-3.1"><t>
   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.</t>

	<t>
   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.</t>

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

	</section>

	<section title="Generic Encoders and Decoders" anchor="section-3.2"><t>
   A generic CBOR decoder can decode all well-formed CBOR data and
   present them to an application.  CBOR data is well-formed if it uses
   the initial bytes, as well as the byte strings and/or data items that
   are implied by their values, in the manner defined by CBOR, and no
   extraneous data follows (Appendix C).</t>

	<t>
   Even though CBOR attempts to minimize these cases, not all well-
   formed CBOR data is 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
   <xref target="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
   codepoints are not registered at the time the encoder/decoder is
   written (<xref target="section-3.5"/>).</t>

	<t>
   Generic decoders provide ways to present well-formed CBOR values,
   both valid and invalid, to an application.  The diagnostic notation
   (<xref target="section-6"/>) may be used to present well-formed CBOR values to humans.</t>

	<t>
   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.</t>

	</section>

	<section title="Syntax Errors" anchor="section-3.3"><t>
   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 a decoder-specific convention that clearly
   indicates there has been a problem, or take some other action.</t>

	<section title="Incomplete CBOR Data Items" anchor="section-3.3.1"><t>
   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 is available.</t>

	<t>
   Examples of incomplete data items include:</t>

	<t><list style="symbols"><t>A decoder expects a certain number of array or map entries but
      instead encounters the end of the data.</t>

	<t>A decoder processes what it expects to be the last pair in a map
      and comes to the end of the data.</t>

	<t>A decoder has just seen a tag and then encounters the end of the
      data.</t>

	<t>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.</t>

	</list>
	</t>

	</section>

	<section title="Malformed Indefinite-Length Items" anchor="section-3.3.2"><t>
   Examples of malformed indefinite-length data items include:</t>

	<t><list style="symbols"><t>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.</t>

	<t>Within an indefinite-length map, a decoder encounters the "break"
      stop code immediately after reading a key (the value is missing).</t>

	</list>
	</t>

	<t>
   Another error is finding a "break" stop code at a point in the data
   where there is no immediately enclosing (unclosed) indefinite-length
   item.</t>

	</section>

	<section title="Unknown Additional Information Values" anchor="section-3.3.3"><t>
   At the time of writing, some additional information values are
   unassigned and reserved for future versions of this document (see
   <xref target="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.</t>

	</section>

	</section>

	<section title="Other Decoding Errors" anchor="section-3.4"><t>
   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.</t>

	<t>
   Such problems might include:</t>

	<t><list style="hanging" hangIndent="3"><t hangText="Duplicate keys in a map:">
	Generic decoders (<xref target="section-3.2"/>) make data
	<vspace blankLines="0"/>
	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 (<xref target="section-3.7"/>).
	</t>

	<t hangText="Inadmissible type on the value following a tag:">
	Tags (<xref target="section-2.4"/>)
	<vspace blankLines="0"/>
	specify what type of data item is supposed to follow the tag; for
      example, the tags for positive or negative bignums are supposed to
      be put on byte strings.  A decoder that decodes the tagged data
      item into a native representation (a native big integer in this
      example) is expected to check the type of the data item being
      tagged.  Even decoders that don't have such native representations
      available in their environment may perform the check on those tags
      known to them and react appropriately.
	</t>

	<t hangText="Invalid UTF-8 string:">
	A decoder might or might not want to verify
	<vspace blankLines="0"/>
	that the sequence of bytes in a UTF-8 string (major type 3) is
      actually valid UTF-8 and react appropriately.
	</t>

	</list>
	</t>

	</section>

	<section title="Handling Unknown Simple Values and Tags" anchor="section-3.5"><t>
   A decoder that comes across a simple value (<xref target="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.</t>

	<t>
   A decoder that comes across a tag (<xref target="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.</t>

	</section>

	<section title="Numbers" anchor="section-3.6"><t>
   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.</t>

	<t>
   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.</t>

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

	<t>
   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.</t>

	<t>
   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.</t>

	</section>

	<section title="Specifying Keys for Maps" anchor="section-3.7"><t>
   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.</t>

	<t>
   If multiple types of keys are to be used, consideration should be
   given to how these types would be represented in the specific
   programming environments that are to be used.  For example, in
   JavaScript 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.</t>

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

	<t>
   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 strict mode
   (<xref target="section-3.10"/>).</t>

	<t>
   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.)</t>

	<t>
   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.</t>

	</section>

	<section title="Undefined Values" anchor="section-3.8"><t>
   In some CBOR-based protocols, the simple value (<xref target="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.</t>

	</section>

	<section title="Canonical CBOR" anchor="section-3.9"><t>
   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.</t>

	<t>
   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:</t>

	<t><list style="symbols"><t>Integers must be as small as possible.<list style="symbols"><t>0 to 23 and -1 to -24 must be expressed in the same byte as the
         major type;</t>

	<t>24 to 255 and -25 to -256 must be expressed only with an
         additional uint8_t;</t>

	<t>256 to 65535 and -257 to -65536 must be expressed only with an
         additional uint16_t;</t>

	<t>65536 to 4294967295 and -65537 to -4294967296 must be expressed
         only with an additional uint32_t.</t>

	</list>
	</t>

	<t>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.</t>

	<t>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:<list style="symbols"><t>If two keys have different lengths, the shorter one sorts
         earlier;</t>

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

	</list>
	</t>

	<t>Indefinite-length items must be made into definite-length items.</t>

	</list>
	</t>

	<t>
   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.</t>

	<t>
   CBOR tags present additional considerations for 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-
   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.</t>

	</section>

	<section title="Strict Mode" anchor="section-3.10"><t>
   Some areas of application of CBOR do not require canonicalization
   (<xref target="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.</t>

	<t>
   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.</t>

	<t>
   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 (<xref target="section-3.3"/>).  It will also
   expend the effort to reliably detect other decoding errors
   (<xref target="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:</t>

	<t><list style="symbols"><t>a map (major type 5) that has more than one entry with the same
      key</t>

	<t>a tag that is used on a data item of the incorrect type</t>

	<t>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</t>

	</list>
	</t>

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

	<t><list style="symbols"><t>It can report an error (and not return data).</t>

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

	</list>
	</t>

	<t>
   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.)</t>

	<t>
   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.</t>

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

	</section>

	</section>

	<section title="Converting Data between CBOR and JSON" anchor="section-4"><t>
   This section gives non-normative advice about converting between CBOR
   and JSON.  Implementations of converters are free to use whichever
   advice here they want.</t>

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

	<section title="Converting from CBOR to JSON" anchor="section-4.1"><t>
   Most of the types in CBOR have direct analogs in JSON.  However, some
   do not, and someone implementing a CBOR-to-JSON converter has to
   consider what to do in those cases.  The following non-normative
   advice deals with these by converting them to a single substitute
   value, such as a JSON null.</t>

	<t><list style="symbols"><t>An integer (major type 0 or 1) becomes a JSON number.</t>

	<t>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.</t>

	<t>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.</t>

	<t>An array (major type 4) becomes a JSON array.</t>

	<t>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.</t>

	<t>False (major type 7, additional information 20) becomes a JSON
      false.</t>

	<t>True (major type 7, additional information 21) becomes a JSON
      true.</t>

	<t>Null (major type 7, additional information 22) becomes a JSON
      null.</t>

	<t>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.</t>

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

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

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

	<t>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.</t>

	<t>Indefinite-length items are made definite before conversion.</t>

	</list>
	</t>

	</section>

	<section title="Converting from JSON to CBOR" anchor="section-4.2"><t>
   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:</t>

	<t><list style="symbols"><t>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.)</t>

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

	</list>
	</t>

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

	</section>

	</section>

	<section title="Future Evolution of CBOR" anchor="section-5"><t>
   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.</t>

	<t>
   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.</t>

	<t>
   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.</t>

	<t>
   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.</t>

	<section title="Extension Points" anchor="section-5.1"><t>
   In a protocol design, opportunities for evolution are often included
   in the form of extension points.  For example, there may be a
   codepoint space that is not fully allocated from the outset, and the
   protocol is designed to tolerate and embrace implementations that
   start using more codepoints than initially allocated.</t>

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

	<t>
   CBOR has three major extension points:</t>

	<t><list style="symbols"><t>the "simple" space (values in major type 7).  Of the 24 efficient
      (and 224 slightly less efficient) values, only a small number have
      been allocated.  Implementations receiving an unknown simple data
      item may be able to process it as such, given that the structure
      of the value is indeed simple.  The IANA registry in <xref target="section-7.1"/>
      is the appropriate way to address the extensibility of this
      codepoint space.</t>

	<t>the "tag" space (values in major type 6).  Again, only a small
      part of the codepoint 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.  The IANA registry in <xref target="section-7.2"/> is the
      appropriate way to address the extensibility of this codepoint
      space.</t>

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

	</list>
	</t>

	</section>

	<section title="Curating the Additional Information Space" anchor="section-5.2"><t>
   The human mind is sometimes drawn to filling in little perceived gaps
   to make something neat.  We expect the remaining gaps in the
   codepoint space for the additional information values to be an
   attractor for new ideas, just because they are there.</t>

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

	<t>
   For an additional information value of n &gt;= 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.</t>

	</section>

	</section>

	<section title="Diagnostic Notation" anchor="section-6"><t>
   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.</t>

	<t>
   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 <xref target="YAML"/>.)</t>

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

	<t>
   The notation borrows the JSON syntax for numbers (integer and
   floating point), True (&gt;true&lt;), False (&gt;false&lt;), Null (&gt;null&lt;), 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 &gt;undefined&lt; 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:</t>

	<t><list hangIndent="3" style="hanging"><t>
      0("2013-03-21T20:04:00Z")</t>

	</list>
	</t>

	<t><list style="hanging" hangIndent="3"><t hangText="or the equivalent relative time as">
	<vspace blankLines="1"/>
	1(1363896240)
	</t>

	</list>
	</t>

	<t>
   Byte strings are notated in one of the base encodings, without
   padding, enclosed in single quotes, prefixed by &gt;h&lt; for base16, &gt;b32&lt; for base32, &gt;h32&lt; for base32hex, &gt;b64&lt; 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'.</t>

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

	<section title="Encoding Indicators" anchor="section-6.1"><t>
   Sometimes it is useful to indicate in the diagnostic notation which
   of several alternative representations were actually used; for
   example, a data item written &gt;1.5&lt; by a diagnostic decoder might have been encoded as a half-, single-, or double-precision float.</t>

	<t>
   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.</t>

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

	<t>
   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.)</t>

	<t>
   As a special case, byte and text strings of indefinite length can be
   notated in the form (_ h'0123', h'4567') and (_ "foo", "bar").</t>

	</section>

	</section>

	<section title="IANA Considerations" anchor="section-7"><t>
   IANA has created two registries for new CBOR values.  The registries
   are separate, that is, not under an umbrella registry, and follow the
   rules in <xref target="RFC5226"/>.  IANA has also assigned a new MIME media type and
   an associated Constrained Application Protocol (CoAP) Content-Format
   entry.</t>

	<section title="Simple Values Registry" anchor="section-7.1"><t>
   IANA has created the "Concise Binary Object Representation (CBOR) Simple Values" registry.  The initial values are shown in Table 2.</t>

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

	<t>
   New entries in the range 32 to 255 are assigned by Specification
   Required.</t>

	</section>

	<section title="Tags Registry" anchor="section-7.2"><t>
   IANA has created the "Concise Binary Object Representation (CBOR) Tags" registry.  The initial values are shown in Table 3.</t>

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

	<t><list style="symbols"><t>Data item</t>

	<t>Semantics (short form)</t>

	</list>
	</t>

	<t>
   In addition, First Come First Served requests should include:</t>

	<t><list style="symbols"><t>Point of contact</t>

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

	</list>
	</t>

	</section>

	<section title="Media Type (&quot;MIME Type&quot;)" anchor="section-7.3"><t>
   The Internet media type <xref target="RFC6838"/> for CBOR data is application/cbor.</t>

	<t>
   Type name: application</t>

	<t>
   Subtype name: cbor</t>

	<t>
   Required parameters: n/a</t>

	<t>
   Optional parameters: n/a</t>

	<t><list style="hanging" hangIndent="-1"><t hangText="Encoding considerations:">
	binary
	<vspace blankLines="0"/>
	</t>

	<t hangText="Security considerations:">
	See Section 8 of this document
	<vspace blankLines="0"/>
	</t>

	</list>
	</t>

	<t>
   Interoperability considerations: n/a</t>

	<t>
   Published specification: This document</t>

	<t><list style="hanging" hangIndent="3"><t hangText="Applications that use this media type:">
	None yet, but it is expected
	<vspace blankLines="0"/>
	that this format will be deployed in protocols and applications.
	</t>

	<t hangText="Additional information:">
	<vspace blankLines="0"/>
	Magic number(s): n/a
      File extension(s): .cbor
      Macintosh file type code(s): n/a
	</t>

	<t hangText="Person &amp; email address to contact for further information:">
	<vspace blankLines="0"/>
	Carsten Bormann
      cabo@tzi.org
	</t>

	</list>
	</t>

	<t>
   Intended usage: COMMON</t>

	<t>
   Restrictions on usage: none</t>

	<t><list style="hanging" hangIndent="3"><t hangText="Author:">
	<vspace blankLines="0"/>
	Carsten Bormann &lt;cabo@tzi.org&gt;
	</t>

	<t hangText="Change controller:">
	<vspace blankLines="0"/>
	The IESG &lt;iesg@ietf.org&gt;
	</t>

	</list>
	</t>

	</section>

	<section title="CoAP Content-Format" anchor="section-7.4"><t>
   Media Type: application/cbor</t>

	<t>
   Encoding: -</t>

	<t>
   Id: 60</t>

	<t>
   Reference: [RFC7049]</t>

	</section>

	<section title="The +cbor Structured Syntax Suffix Registration" anchor="section-7.5"><t>
   Name: Concise Binary Object Representation (CBOR)</t>

	<t>
   +suffix: +cbor</t>

	<t>
   References: [RFC7049]</t>

	<t>
   Encoding Considerations: CBOR is a binary format.</t>

	<t>
   Interoperability Considerations: n/a</t>

	<t><list style="hanging" hangIndent="3"><t hangText="Fragment Identifier Considerations:">
	<vspace blankLines="0"/>
	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".)
	<vspace blankLines="1"/>
	The syntax and semantics for fragment identifiers for a specific
      "xxx/yyy+cbor" SHOULD be processed as follows:
	<vspace blankLines="1"/>
	For cases defined in +cbor, where the fragment identifier resolves
      per the +cbor rules, then process as specified in +cbor.
	<vspace blankLines="1"/>
	For cases defined in +cbor, where the fragment identifier does not
      resolve per the +cbor rules, then process as specified in
      "xxx/yyy+cbor".
	<vspace blankLines="1"/>
	For cases not defined in +cbor, then process as specified in
      "xxx/yyy+cbor".
	</t>

	<t hangText="Security Considerations:">
	See Section 8 of this document
	<vspace blankLines="0"/>
	</t>

	<t hangText="Contact:">
	<vspace blankLines="0"/>
	Apps Area Working Group (apps-discuss@ietf.org)
	</t>

	<t hangText="Author/Change Controller:">
	<vspace blankLines="0"/>
	The Apps Area Working Group.
      The IESG has change control over this registration.
	</t>

	</list>
	</t>

	</section>

	</section>

	<section title="Security Considerations" anchor="section-8"><t>
   A network-facing application can exhibit vulnerabilities in its
   processing logic for incoming data.  Complex parsers are well known
   as a likely source of such vulnerabilities, such as the ability to
   remotely crash a node, or even remotely execute arbitrary code on it.
   CBOR attempts to narrow the opportunities for introducing such
   vulnerabilities by reducing parser complexity, by giving the entire
   range of encodable values a meaning where possible.</t>

	<t>
   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.)</t>

	<t>
   Applications where a CBOR data item is 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 (<xref target="section-3.10"/>).</t>

	</section>

	<section title="Acknowledgements" anchor="section-9"><t>
   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.</t>

	<t>
   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</t>

	<t>
   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.</t>

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

	<t>
   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.</t>

	</section>

	</middle>

	<back>
	<references title="Normative References">
	<reference anchor="ECMA262" target="http://www.ecma-international.org/publications/files/ecma-st/ECMA-262.pdf"><front>
	<title>ECMAScript Language Specification 5.1 Edition</title>
	<author>
	<organization>European Computer Manufacturers Association</organization>
	</author>

	<date month="June" year="2011"/>
	</front>

	<seriesInfo name="ECMA" value="Standard ECMA-262"/>
	</reference>
	&RFC2045;
	&RFC2119;
	&RFC3339;
	&RFC3629;
	&RFC3986;
	&RFC4287;
	&RFC4648;
	&RFC5226;
	<reference><front>
	<!--
   rfc7049.txt(2191): Warning: Failed parsing a reference.  Are all elements
   separated by commas (not periods, not just spaces)?:
   [TIME_T]   The Open Group Base Specifications, "Vol. 1: Base
              Definitions, Issue 7", Section 4.15 'Seconds Since the
              Epoch', IEEE Std 1003.1, 2013 Edition, 2013,
              <http://pubs.opengroup.org/onlinepubs/9699919799/
              basedefs/V1_chap04.html#tag_04_15>.
   --></front>

	</reference>
	</references>
	<references title="Informative References">
	<reference anchor="ASN.1"><front>
	<title>Information Technology -- ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)</title>
	<author>
	<organization>International Telecommunication Union</organization>
	</author>

	<date year="1994"/>
	</front>

	<seriesInfo name="ITU-T" value="Recommendation X.690"/>
	</reference>
	<reference anchor="BSON" target="http://bsonspec.org/"><front>
	<title>BSON - Binary JSON</title>
	<author>
	<organization>Various</organization>
	</author>

	<date year="2013"/>
	</front>

	</reference>
	<reference anchor="CNN-TERMS"><front>
	<title>Terminology for Constrained Node Networks</title>
	<author fullname="C. Bormann" initials="C." surname="Bormann">
	</author>

	<author fullname="M. Ersue" initials="M." surname="Ersue">
	</author>

	<author fullname="A. Keranen" initials="A." surname="Keranen">
	</author>

	<date month="July" year="2013"/>
	</front>

	<seriesInfo name="Work" value="in Progress"/>
	</reference>
	<reference anchor="MessagePack" target="http://msgpack.org/"><front>
	<title>MessagePack</title>
	<author fullname="S. Furuhashi" initials="S." surname="Furuhashi">
	</author>

	<date year="2013"/>
	</front>

	</reference>
	&RFC0713;
	&RFC4627;
	&RFC6838;
	<reference anchor="UBJSON" target="http://ubjson.org/"><front>
	<title>Universal Binary JSON Specification</title>
	<author>
	<organization>The Buzz Media</organization>
	</author>

	<date year="2013"/>
	</front>

	</reference>
	<reference anchor="YAML" target="http://www.yaml.org/spec/1.2/spec.html"><front>
	<title>YAML Ain't Markup Language (YAML[TM]) Version 1.2</title>
	<author fullname="O. Ben-Kiki" initials="O." surname="Ben-Kiki">
	</author>

	<author fullname="C. Evans" initials="C." surname="Evans">
	</author>

	<author fullname="I. Net" initials="I." surname="Net">
	</author>

	<date month="October" year="2009"/>
	</front>

	<seriesInfo name="3rd" value="Edition"/>
	</reference>
	</references>
	<section title="Examples" anchor="section-a"><t>
   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')).</t>

	<texttable title="Examples of Encoded CBOR Data Items" anchor="ref-examples-of-encoded-cbor-data-items" style="full"><ttcol> Diagnostic</ttcol>
	<ttcol> Encoded</ttcol>
	<c>0</c>
	<c>0x00</c>
	<c>1</c>
	<c>0x01</c>
	<c>10</c>
	<c>0x0a</c>
	<c>23</c>
	<c>0x17</c>
	<c>24</c>
	<c>0x1818</c>
	<c>25</c>
	<c>0x1819</c>
	<c>100</c>
	<c>0x1864</c>
	<c>1000</c>
	<c>0x1903e8</c>
	<c>1000000</c>
	<c>0x1a000f4240</c>
	<c>1000000000000</c>
	<c>0x1b000000e8d4a51000</c>
	<c>18446744073709551615</c>
	<c>0x1bffffffffffffffff</c>
	<c>18446744073709551616</c>
	<c>0xc249010000000000000000</c>
	<c>-18446744073709551616</c>
	<c>0x3bffffffffffffffff</c>
	<c>-18446744073709551617</c>
	<c>0xc349010000000000000000</c>
	<c>-1</c>
	<c>0x20</c>
	<c>-10</c>
	<c>0x29</c>
	<c>-100</c>
	<c>0x3863</c>
	<c>-1000</c>
	<c>0x3903e7</c>
	<c>0.0</c>
	<c>0xf90000</c>
	<c>-0.0</c>
	<c>0xf98000</c>
	<c>1.0</c>
	<c>0xf93c00</c>
	<c>1.1</c>
	<c>0xfb3ff199999999999a</c>
	<c>1.5</c>
	<c>0xf93e00</c>
	<c>65504.0</c>
	<c>0xf97bff</c>
	<c>100000.0</c>
	<c>0xfa47c35000</c>
	<c>3.4028234663852886e+38</c>
	<c>0xfa7f7fffff</c>
	<c>1.0e+300</c>
	<c>0xfb7e37e43c8800759c</c>
	<c>5.960464477539063e-8</c>
	<c>0xf90001</c>
	<c>0.00006103515625</c>
	<c>0xf90400</c>
	<c>-4.0</c>
	<c>0xf9c400</c>
	<c>-4.1</c>
	<c>0xfbc010666666666666</c>
	<c>Infinity</c>
	<c>0xf97c00</c>
	<c>NaN</c>
	<c>0xf97e00</c>
	<c>-Infinity</c>
	<c>0xf9fc00</c>
	<c>Infinity</c>
	<c>0xfa7f800000</c>
	<c>NaN</c>
	<c>0xfa7fc00000</c>
	<c>-Infinity</c>
	<c>0xfaff800000</c>
	<c>Infinity</c>
	<c>0xfb7ff0000000000000</c>
	<c>NaN</c>
	<c>0xfb7ff8000000000000</c>
	<c>-Infinity</c>
	<c>0xfbfff0000000000000</c>
	<c>false</c>
	<c>0xf4</c>
	<c>true</c>
	<c>0xf5</c>
	<c>null</c>
	<c>0xf6</c>
	<c>undefined</c>
	<c>0xf7</c>
	<c>simple(16)</c>
	<c>0xf0</c>
	<c>simple(24)</c>
	<c>0xf818</c>
	<c>simple(255)</c>
	<c>0xf8ff</c>
	<c>0("2013-03-21T20:04:00Z")</c>
	<c>0xc074323031332d30332d32315432303a</c>
	<c></c>
	<c>30343a30305a</c>
	<c>1(1363896240)</c>
	<c>0xc11a514b67b0</c>
	<c>1(1363896240.5)</c>
	<c>0xc1fb41d452d9ec200000</c>
	<c>23(h'01020304')</c>
	<c>0xd74401020304</c>
	<c>24(h'6449455446')</c>
	<c>0xd818456449455446</c>
	<c>32("http://www.example.com")</c>
	<c>0xd82076687474703a2f2f7777772e6578</c>
	<c></c>
	<c>616d706c652e636f6d</c>
	<c>h''</c>
	<c>0x40</c>
	<c>h'01020304'</c>
	<c>0x4401020304</c>
	<c>""</c>
	<c>0x60</c>
	<c>"a"</c>
	<c>0x6161</c>
	<c>"IETF"</c>
	<c>0x6449455446</c>
	<c>"\"\\"</c>
	<c>0x62225c</c>
	<c>"\u00fc"</c>
	<c>0x62c3bc</c>
	<c>"\u6c34"</c>
	<c>0x63e6b0b4</c>
	<c>"\ud800\udd51"</c>
	<c>0x64f0908591</c>
	<c>[]</c>
	<c>0x80</c>
	<c>[1, 2, 3]</c>
	<c>0x83010203</c>
	<c>[1, [2, 3], [4, 5]]</c>
	<c>0x8301820203820405</c>
	<c>[1, 2, 3, 4, 5, 6, 7, 8, 9,</c>
	<c>0x98190102030405060708090a0b0c0d0e</c>
	<c>10, 11, 12, 13, 14, 15, 16,</c>
	<c>0f101112131415161718181819</c>
	<c>17, 18, 19, 20, 21, 22, 23,</c>
	<c></c>
	<c>24, 25]</c>
	<c></c>
	<c>{}</c>
	<c>0xa0</c>
	<c>{1: 2, 3: 4}</c>
	<c>0xa201020304</c>
	<c>{"a": 1, "b": [2, 3]}</c>
	<c>0xa26161016162820203</c>
	<c>["a", {"b": "c"}]</c>
	<c>0x826161a161626163</c>
	<c>{"a": "A", "b": "B", "c":</c>
	<c>0xa5616161416162614261636143616461</c>
	<c>"C", "d": "D", "e": "E"}</c>
	<c>4461656145</c>
	<c>(_ h'0102', h'030405')</c>
	<c>0x5f42010243030405ff</c>
	<c>(_ "strea", "ming")</c>
	<c>0x7f657374726561646d696e67ff</c>
	<c>[_ ]</c>
	<c>0x9fff</c>
	<c>[_ 1, [2, 3], [_ 4, 5]]</c>
	<c>0x9f018202039f0405ffff</c>
	<c>[_ 1, [2, 3], [4, 5]]</c>
	<c>0x9f01820203820405ff</c>
	<c>[1, [2, 3], [_ 4, 5]]</c>
	<c>0x83018202039f0405ff</c>
	<c>[1, [_ 2, 3], [4, 5]]</c>
	<c>0x83019f0203ff820405</c>
	<c>[_ 1, 2, 3, 4, 5, 6, 7, 8,</c>
	<c>0x9f0102030405060708090a0b0c0d0e0f</c>
	<c>9, 10, 11, 12, 13, 14, 15,</c>
	<c>101112131415161718181819ff</c>
	<c>16, 17, 18, 19, 20, 21, 22,</c>
	<c></c>
	<c>23, 24, 25]</c>
	<c></c>
	<c>{_ "a": 1, "b": [_ 2, 3]}</c>
	<c>0xbf61610161629f0203ffff</c>
	<c>["a", {_ "b": "c"}]</c>
	<c>0x826161bf61626163ff</c>
	<c>{_ "Fun": true, "Amt": -2}</c>
	<c>0xbf6346756ef563416d7421ff</c>
	</texttable>
	</section>

	<section title="Jump Table" anchor="section-b"><t>
   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.)<!--
   rfc7049.txt(2650): Warning: Unexpected title: expected 'Figure ...', found
   'Table 5: Jump Table for Initial Byte'.  This looks like a figure that has
   been entered as a texttable.  The generated XML will need adjustment.
   --></t>

	<figure title="Jump Table for Initial Byte" anchor="ref-jump-table-for-initial-byte"><artwork><![CDATA[
+-----------------+-------------------------------------------------+
| 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)                            |
|                 |                                                 |
| 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      |
|                 | 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)                                   |
|                 |                                                 |
| 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)                                  |
|                 |                                                 |
| 0xf4            | False                                           |
|                 |                                                 |
| 0xf5            | True                                            |
|                 |                                                 |
| 0xf6            | Null                                            |
|                 |                                                 |
| 0xf7            | Undefined                                       |
|                 |                                                 |
| 0xf8            | (simple value, one byte follows)                |
|                 |                                                 |
| 0xf9            | Half-Precision Float (two-byte IEEE 754)        |
|                 |                                                 |
| 0xfa            | Single-Precision Float (four-byte IEEE 754)     |
|                 |                                                 |
| 0xfb            | Double-Precision Float (eight-byte IEEE 754)    |
|                 |                                                 |
| 0xff            | "break" stop code                               |
+-----------------+-------------------------------------------------+
]]></artwork>
	</figure>
	</section>

	<section title="Pseudocode" anchor="section-c"><t>
   The well-formedness of a CBOR item can be checked by the pseudocode
   in Figure 1.  The data is well-formed if and only if:</t>

	<t><list style="symbols"><t>the pseudocode does not "fail";</t>

	<t>after execution of the pseudocode, no bytes are left in the input
      (except in streaming applications)</t>

	</list>
	</t>

	<t>
   The pseudocode has the following prerequisites:</t>

	<t><list style="symbols"><t>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.</t>

	<t>uint() converts a byte string into an unsigned integer by
      interpreting the byte string in network byte order.</t>

	<t>Arithmetic works as in C.</t>

	<t>All variables are unsigned integers of sufficient range.</t>

	</list>
	</t>

	<figure title="Pseudocode for Well-Formedness Check" anchor="ref-pseudocode-for-well-formedness-check"><artwork><![CDATA[
well_formed (breakable = false) {
  // process initial bytes
  ib = uint(take(1));
  mt = ib >> 5;
  val = ai = ib & 0x1f;
  switch (ai) {
    case 24: val = uint(take(1)); break;
    case 25: val = uint(take(2)); break;
    case 26: val = uint(take(4)); break;
    case 27: val = uint(take(8)); break;
    case 28: case 29: case 30: fail();
    case 31:
      return well_formed_indefinite(mt, breakable);
  }
  // process content
  switch (mt) {
    // case 0, 1, 7 do not have content; just use val
    case 2: case 3: take(val); break; // bytes/UTF-8
    case 4: for (i = 0; i < val; i++) well_formed(); break;
    case 5: for (i = 0; i < val*2; i++) well_formed(); break;
    case 6: well_formed(); break;     // 1 embedded data item
  }
  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
}
]]></artwork>
	</figure>
	<t>
   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.</t>

	<t>
   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).</t>

	<figure title="Pseudocode for Encoding a Signed Integer" anchor="ref-pseudocode-for-encoding-a-signed-integer"><artwork><![CDATA[
void encode_sint(int64_t n) {
  uint64t ui = n >> 63;    // extend sign to whole length
  mt = ui & 0x20;          // extract major type
  ui ^= n;                 // complement negatives
  if (ui < 24)
    *p++ = mt + ui;
  else if (ui < 256) {
    *p++ = mt + 24;
    *p++ = ui;
  } else
       ...
]]></artwork>
	</figure>
	</section>

	<section title="Half-Precision" anchor="section-d"><t>
   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).</t>

	<t>
   #include &lt;math.h&gt;</t>

	<figure title="C Code for a Half-Precision Decoder" anchor="ref-c-code-for-a-half-precision-decoder"><artwork><![CDATA[
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;
}
]]></artwork>
	</figure>
	<figure><artwork><![CDATA[
import struct
from math import ldexp

def decode_single(single):
    return struct.unpack("!f", struct.pack("!I", single))[0]
]]></artwork>
	</figure>
	<figure><artwork><![CDATA[
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
]]></artwork>
	</figure>
	</section>

	<section title="Comparison of Other Binary Formats to CBOR's Design Objectives" anchor="section-e"><t>
   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.</t>

	<t>
   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.</t>

	<t>
   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 <xref target="section-1.1"/> is:</t>

	<t><list style="numbers"><t>unambiguous encoding of most common data formats from Internet
       standards</t>

	<t>code compactness for encoder or decoder</t>

	<t>no schema description needed</t>

	<t>reasonably compact serialization</t>

	<t>applicability to constrained and unconstrained applications</t>

	<t>good JSON conversion</t>

	<t>extensibility</t>

	</list>
	</t>

	<section title="ASN.1 DER, BER, and PER" anchor="section-e.1"><t>
   <xref target="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.</t>

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

	</section>

	<section title="MessagePack" anchor="section-e.2"><t>
   <xref target="MessagePack"/> is a concise, widely implemented counted binary
   serialization format, similar in many properties to CBOR, although
   somewhat less regular.  While the data model can be used to represent
   JSON data, MessagePack has also been used in many remote procedure
   call (RPC) applications and for long-term storage of data.</t>

	<t>
   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.</t>

	</section>

	<section title="BSON" anchor="section-e.3"><t>
   <xref target="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.</t>

	</section>

	<section title="UBJSON" anchor="section-e.4"><t>
   <xref target="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
   reserved "unknown-length" value to support streaming of arrays and
   maps (JSON objects).  Within these containers, UBJSON also has a
   "Noop" type for padding.</t>

	</section>

	<section title="MSDTP: RFC 713" anchor="section-e.5"><t>
   Message Services Data Transmission (MSDTP) is a very early example of
   a compact message format; it is described in <xref target="RFC0713"/>, written in
   1976.  It is included here for its historical value, not because it
   was ever widely used.</t>

	</section>

	<section title="Conciseness on the Wire" anchor="section-e.6"><t>
   While CBOR's design objective of code compactness for encoders and
   decoders is a higher priority than its objective of conciseness on
   the wire, many people focus on the wire size.  Table 6 shows some
   encoding examples for the simple nested array [1, [2, 3]]; where some
   form of indefinite-length encoding is supported by the encoding,
   [_ 1, [2, 3]] (indefinite length on the outer array) is also shown.</t>

	<texttable title="Examples for Different Levels of Conciseness" anchor="ref-examples-for-different-levels-of-conciseness" style="full"><ttcol> Format</ttcol>
	<ttcol> [1, [2, 3]]</ttcol>
	<ttcol> [_ 1, [2, 3]]</ttcol>
	<c>RFC 713</c>
	<c>c2 05 81 c2 02 82 83</c>
	<c></c>
	<c>ASN.1 BER</c>
	<c>30 0b 02 01 01 30 06 02</c>
	<c>30 80 02 01 01 30 06 02</c>
	<c></c>
	<c>01 02 02 01 03</c>
	<c>01 02 02 01 03 00 00</c>
	<c>MessagePack</c>
	<c>92 01 92 02 03</c>
	<c></c>
	<c>BSON</c>
	<c>22 00 00 00 10 30 00 01</c>
	<c></c>
	<c></c>
	<c>00 00 00 04 31 00 13 00</c>
	<c></c>
	<c></c>
	<c>00 00 10 30 00 02 00 00</c>
	<c></c>
	<c></c>
	<c>00 10 31 00 03 00 00 00</c>
	<c></c>
	<c></c>
	<c>00 00</c>
	<c></c>
	<c>UBJSON</c>
	<c>61 02 42 01 61 02 42 02</c>
	<c>61 ff 42 01 61 02 42 02</c>
	<c></c>
	<c>42 03</c>
	<c>42 03 45</c>
	<c>CBOR</c>
	<c>82 01 82 02 03</c>
	<c>9f 01 82 02 03 ff</c>
	</texttable>
	</section>

	</section>

	</back>

	</rfc>