| Rfc | 7049 |
|---|
| Title | Concise Binary Object Representation (CBOR) |
|---|
| Author | C. Bormann, P. Hoffman |
|---|
| Date | October 2013 |
|---|
| Format: | TXT, HTML |
|---|
| Obsoleted by | RFC8949 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) C. Bormann
Request for Comments: 7049 Universitaet Bremen TZI
Category: Standards Track P. Hoffman
ISSN: 2070-1721 VPN Consortium
October 2013
Concise Binary Object Representation (CBOR)
Abstract
The Concise Binary Object Representation (CBOR) is a data format
whose design goals include the possibility of extremely small code
size, fairly small message size, and extensibility without the need
for version negotiation. These design goals make it different from
earlier binary serializations such as ASN.1 and MessagePack.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7049.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Specification of the CBOR Encoding . . . . . . . . . . . . . 6
2.1. Major Types . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Indefinite Lengths for Some Major Types . . . . . . . . . 9
2.2.1. Indefinite-Length Arrays and Maps . . . . . . . . . . 9
2.2.2. Indefinite-Length Byte Strings and Text Strings . . . 11
2.3. Floating-Point Numbers and Values with No Content . . . . 12
2.4. Optional Tagging of Items . . . . . . . . . . . . . . . . 14
2.4.1. Date and Time . . . . . . . . . . . . . . . . . . . . 16
2.4.2. Bignums . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.3. Decimal Fractions and Bigfloats . . . . . . . . . . . 17
2.4.4. Content Hints . . . . . . . . . . . . . . . . . . . . 18
2.4.4.1. Encoded CBOR Data Item . . . . . . . . . . . . . 18
2.4.4.2. Expected Later Encoding for CBOR-to-JSON
Converters . . . . . . . . . . . . . . . . . . . 18
2.4.4.3. Encoded Text . . . . . . . . . . . . . . . . . . 19
2.4.5. Self-Describe CBOR . . . . . . . . . . . . . . . . . 19
3. Creating CBOR-Based Protocols . . . . . . . . . . . . . . . . 20
3.1. CBOR in Streaming Applications . . . . . . . . . . . . . 20
3.2. Generic Encoders and Decoders . . . . . . . . . . . . . . 21
3.3. Syntax Errors . . . . . . . . . . . . . . . . . . . . . . 21
3.3.1. Incomplete CBOR Data Items . . . . . . . . . . . . . 22
3.3.2. Malformed Indefinite-Length Items . . . . . . . . . . 22
3.3.3. Unknown Additional Information Values . . . . . . . . 23
3.4. Other Decoding Errors . . . . . . . . . . . . . . . . . . 23
3.5. Handling Unknown Simple Values and Tags . . . . . . . . . 24
3.6. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.7. Specifying Keys for Maps . . . . . . . . . . . . . . . . 25
3.8. Undefined Values . . . . . . . . . . . . . . . . . . . . 26
3.9. Canonical CBOR . . . . . . . . . . . . . . . . . . . . . 26
3.10. Strict Mode . . . . . . . . . . . . . . . . . . . . . . . 28
4. Converting Data between CBOR and JSON . . . . . . . . . . . . 29
4.1. Converting from CBOR to JSON . . . . . . . . . . . . . . 29
4.2. Converting from JSON to CBOR . . . . . . . . . . . . . . 30
5. Future Evolution of CBOR . . . . . . . . . . . . . . . . . . 31
5.1. Extension Points . . . . . . . . . . . . . . . . . . . . 32
5.2. Curating the Additional Information Space . . . . . . . . 33
6. Diagnostic Notation . . . . . . . . . . . . . . . . . . . . . 33
6.1. Encoding Indicators . . . . . . . . . . . . . . . . . . . 34
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
7.1. Simple Values Registry . . . . . . . . . . . . . . . . . 35
7.2. Tags Registry . . . . . . . . . . . . . . . . . . . . . . 35
7.3. Media Type ("MIME Type") . . . . . . . . . . . . . . . . 36
7.4. CoAP Content-Format . . . . . . . . . . . . . . . . . . . 37
7.5. The +cbor Structured Syntax Suffix Registration . . . . . 37
8. Security Considerations . . . . . . . . . . . . . . . . . . . 38
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
10.1. Normative References . . . . . . . . . . . . . . . . . . 39
10.2. Informative References . . . . . . . . . . . . . . . . . 40
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 41
Appendix B. Jump Table . . . . . . . . . . . . . . . . . . . . . 45
Appendix C. Pseudocode . . . . . . . . . . . . . . . . . . . . . 48
Appendix D. Half-Precision . . . . . . . . . . . . . . . . . . . 50
Appendix E. Comparison of Other Binary Formats to CBOR's Design
Objectives . . . . . . . . . . . . . . . . . . . . . 51
E.1. ASN.1 DER, BER, and PER . . . . . . . . . . . . . . . . . 52
E.2. MessagePack . . . . . . . . . . . . . . . . . . . . . . . 52
E.3. BSON . . . . . . . . . . . . . . . . . . . . . . . . . . 53
E.4. UBJSON . . . . . . . . . . . . . . . . . . . . . . . . . 53
E.5. MSDTP: RFC 713 . . . . . . . . . . . . . . . . . . . . . 53
E.6. Conciseness on the Wire . . . . . . . . . . . . . . . . . 53
1. Introduction
There are hundreds of standardized formats for binary representation
of structured data (also known as binary serialization formats). Of
those, some are for specific domains of information, while others are
generalized for arbitrary data. In the IETF, probably the best-known
formats in the latter category are ASN.1's BER and DER [ASN.1].
The format defined here follows some specific design goals that are
not well met by current formats. The underlying data model is an
extended version of the JSON data model [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.
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).
1.1. Objectives
The objectives of CBOR, roughly in decreasing order of importance,
are:
1. The representation must be able to unambiguously encode most
common data formats used in Internet standards.
* It must represent a reasonable set of basic data types and
structures using binary encoding. "Reasonable" here is
largely influenced by the capabilities of JSON, with the major
addition of binary byte strings. The structures supported are
limited to arrays and trees; loops and lattice-style graphs
are not supported.
* There is no requirement that all data formats be uniquely
encoded; that is, it is acceptable that the number "7" might
be encoded in multiple different ways.
2. The code for an encoder or decoder must be able to be compact in
order to support systems with very limited memory, processor
power, and instruction sets.
* An encoder and a decoder need to be implementable in a very
small amount of code (for example, in class 1 constrained
nodes as defined in [CNN-TERMS]).
* The format should use contemporary machine representations of
data (for example, not requiring binary-to-decimal
conversion).
3. Data must be able to be decoded without a schema description.
* Similar to JSON, encoded data should be self-describing so
that a generic decoder can be written.
4. The serialization must be reasonably compact, but data
compactness is secondary to code compactness for the encoder and
decoder.
* "Reasonable" here is bounded by JSON as an upper bound in
size, and by implementation complexity maintaining a lower
bound. Using either general compression schemes or extensive
bit-fiddling violates the complexity goals.
5. The format must be applicable to both constrained nodes and high-
volume applications.
* This means it must be reasonably frugal in CPU usage for both
encoding and decoding. This is relevant both for constrained
nodes and for potential usage in applications with a very high
volume of data.
6. The format must support all JSON data types for conversion to and
from JSON.
* It must support a reasonable level of conversion as long as
the data represented is within the capabilities of JSON. It
must be possible to define a unidirectional mapping towards
JSON for all types of data.
7. The format must be extensible, and the extended data must be
decodable by earlier decoders.
* The format is designed for decades of use.
* The format must support a form of extensibility that allows
fallback so that a decoder that does not understand an
extension can still decode the message.
* The format must be able to be extended in the future by later
IETF standards.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119, BCP 14
[RFC2119] and indicate requirement levels for compliant CBOR
implementations.
The term "byte" is used in its now-customary sense as a synonym for
"octet". All multi-byte values are encoded in network byte order
(that is, most significant byte first, also known as "big-endian").
This specification makes use of the following terminology:
Data item: A single piece of CBOR data. The structure of a data
item may contain zero, one, or more nested data items. The term
is used both for the data item in representation format and for
the abstract idea that can be derived from that by a decoder.
Decoder: A process that decodes a CBOR data item and makes it
available to an application. Formally speaking, a decoder
contains a parser to break up the input using the syntax rules of
CBOR, as well as a semantic processor to prepare the data in a
form suitable to the application.
Encoder: A process that generates the representation format of a
CBOR data item from application information.
Data Stream: A sequence of zero or more data items, not further
assembled into a larger containing data item. The independent
data items that make up a data stream are sometimes also referred
to as "top-level data items".
Well-formed: A data item that follows the syntactic structure of
CBOR. A well-formed data item uses the initial bytes and the byte
strings and/or data items that are implied by their values as
defined in CBOR and is not followed by extraneous data.
Valid: A data item that is well-formed and also follows the semantic
restrictions that apply to CBOR data items.
Stream decoder: A process that decodes a data stream and makes each
of the data items in the sequence available to an application as
they are received.
Where bit arithmetic or data types are explained, this document uses
the notation familiar from the programming language C, except that
"**" denotes exponentiation. Similar to the "0x" notation for
hexadecimal numbers, numbers in binary notation are prefixed with
"0b". Underscores can be added to such a number solely for
readability, so 0b00100001 (0x21) might be written 0b001_00001 to
emphasize the desired interpretation of the bits in the byte; in this
case, it is split into three bits and five bits.
2. Specification of the CBOR Encoding
A CBOR-encoded data item is structured and encoded as described in
this section. The encoding is summarized in Table 5.
The initial byte of each data item contains both information about
the major type (the high-order 3 bits, described in Section 2.1) and
additional information (the low-order 5 bits). When the value of the
additional information is less than 24, it is directly used as a
small unsigned integer. When it is 24 to 27, the additional bytes
for a variable-length integer immediately follow; the values 24 to 27
of the additional information specify that its length is a 1-, 2-,
4-, or 8-byte unsigned integer, respectively. Additional information
value 31 is used for indefinite-length items, described in
Section 2.2. Additional information values 28 to 30 are reserved for
future expansion.
In all additional information values, the resulting integer is
interpreted depending on the major type. It may represent the actual
data: for example, in integer types, the resulting integer is used
for the value itself. It may instead supply length information: for
example, in byte strings it gives the length of the byte string data
that follows.
A CBOR decoder implementation can be based on a jump table with all
256 defined values for the initial byte (Table 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).
2.1. Major Types
The following lists the major types and the additional information
and other bytes associated with the type.
Major type 0: an unsigned integer. The 5-bit additional information
is either the integer itself (for additional information values 0
through 23) or the length of additional data. Additional
information 24 means the value is represented in an additional
uint8_t, 25 means a uint16_t, 26 means a uint32_t, and 27 means a
uint64_t. For example, the integer 10 is denoted as the one byte
0b000_01010 (major type 0, additional information 10). The
integer 500 would be 0b000_11001 (major type 0, additional
information 25) followed by the two bytes 0x01f4, which is 500 in
decimal.
Major type 1: a negative integer. The encoding follows the rules
for unsigned integers (major type 0), except that the value is
then -1 minus the encoded unsigned integer. For example, the
integer -500 would be 0b001_11001 (major type 1, additional
information 25) followed by the two bytes 0x01f3, which is 499 in
decimal.
Major type 2: a byte string. The string's length in bytes is
represented following the rules for positive integers (major type
0). For example, a byte string whose length is 5 would have an
initial byte of 0b010_00101 (major type 2, additional information
5 for the length), followed by 5 bytes of binary content. A byte
string whose length is 500 would have 3 initial bytes of
0b010_11001 (major type 2, additional information 25 to indicate a
two-byte length) followed by the two bytes 0x01f4 for a length of
500, followed by 500 bytes of binary content.
Major type 3: a text string, specifically a string of Unicode
characters that is encoded as UTF-8 [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").
Major type 4: an array of data items. Arrays are also called lists,
sequences, or tuples. The array's length follows the rules for
byte strings (major type 2), except that the length denotes the
number of data items, not the length in bytes that the array takes
up. Items in an array do not need to all be of the same type.
For example, an array that contains 10 items of any type would
have an initial byte of 0b100_01010 (major type of 4, additional
information of 10 for the length) followed by the 10 remaining
items.
Major type 5: a map of pairs of data items. Maps are also called
tables, dictionaries, hashes, or objects (in JSON). A map is
comprised of pairs of data items, each pair consisting of a key
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
Section 3.7.
Major type 6: optional semantic tagging of other major types. See
Section 2.4.
Major type 7: floating-point numbers and simple data types that need
no content, as well as the "break" stop code. See Section 2.3.
These eight major types lead to a simple table showing which of the
256 possible values for the initial byte of a data item are used
(Table 5).
In major types 6 and 7, many of the possible values are reserved for
future specification. See Section 7 for more information on these
values.
2.2. Indefinite Lengths for Some Major Types
Four CBOR items (arrays, maps, byte strings, and text strings) can be
encoded with an indefinite length using additional information value
31. This is useful if the encoding of the item needs to begin before
the number of items inside the array or map, or the total length of
the string, is known. (The application of this is often referred to
as "streaming" within a data item.)
Indefinite-length arrays and maps are dealt with differently than
indefinite-length byte strings and text strings.
2.2.1. Indefinite-Length Arrays and Maps
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.
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.
For example, assume an encoder wants to represent the abstract array
[1, [2, 3], [4, 5]]. The definite-length encoding would be
0x8301820203820405:
83 -- Array of length 3
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
82 -- Array of length 2
04 -- 4
05 -- 5
Indefinite-length encoding could be applied independently to each of
the three arrays encoded in this data item, as required, leading to
representations such as:
0x9f018202039f0405ffff
9F -- Start indefinite-length array
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
9F -- Start indefinite-length array
04 -- 4
05 -- 5
FF -- "break" (inner array)
FF -- "break" (outer array)
0x9f01820203820405ff
9F -- Start indefinite-length array
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
82 -- Array of length 2
04 -- 4
05 -- 5
FF -- "break"
0x83018202039f0405ff
83 -- Array of length 3
01 -- 1
82 -- Array of length 2
02 -- 2
03 -- 3
9F -- Start indefinite-length array
04 -- 4
05 -- 5
FF -- "break"
0x83019f0203ff820405
83 -- Array of length 3
01 -- 1
9F -- Start indefinite-length array
02 -- 2
03 -- 3
FF -- "break"
82 -- Array of length 2
04 -- 4
05 -- 5
An example of an indefinite-length map (that happens to have two
key/value pairs) might be:
0xbf6346756ef563416d7421ff
BF -- Start indefinite-length map
63 -- First key, UTF-8 string length 3
46756e -- "Fun"
F5 -- First value, true
63 -- Second key, UTF-8 string length 3
416d74 -- "Amt"
21 -- -2
FF -- "break"
2.2.2. Indefinite-Length Byte Strings and Text Strings
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.
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.
For example, assume the sequence:
0b010_11111 0b010_00100 0xaabbccdd 0b010_00011 0xeeff99 0b111_11111
5F -- Start indefinite-length byte string
44 -- Byte string of length 4
aabbccdd -- Bytes content
43 -- Byte string of length 3
eeff99 -- Bytes content
FF -- "break"
After decoding, this results in a single byte string with seven
bytes: 0xaabbccddeeff99.
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.
2.3. Floating-Point Numbers and Values with No Content
Major type 7 is for two types of data: floating-point numbers and
"simple values" that do not need any content. Each value of the
5-bit additional information in the initial byte has its own separate
meaning, as defined in Table 1. Like the major types for integers,
items of this major type do not carry content data; all the
information is in the initial bytes.
+-------------+--------------------------------------------------+
| 5-Bit Value | Semantics |
+-------------+--------------------------------------------------+
| 0..23 | Simple value (value 0..23) |
| | |
| 24 | Simple value (value 32..255 in following byte) |
| | |
| 25 | IEEE 754 Half-Precision Float (16 bits follow) |
| | |
| 26 | IEEE 754 Single-Precision Float (32 bits follow) |
| | |
| 27 | IEEE 754 Double-Precision Float (64 bits follow) |
| | |
| 28-30 | (Unassigned) |
| | |
| 31 | "break" stop code for indefinite-length items |
+-------------+--------------------------------------------------+
Table 1: Values for Additional Information in Major Type 7
As with all other major types, the 5-bit value 24 signifies a single-
byte extension: it is followed by an additional byte to represent the
simple value. (To minimize confusion, only the values 32 to 255 are
used.) This maintains the structure of the initial bytes: as for the
other major types, the length of these always depends on the
additional information in the first byte. Table 2 lists the values
assigned and available for simple types.
+---------+-----------------+
| Value | Semantics |
+---------+-----------------+
| 0..19 | (Unassigned) |
| | |
| 20 | False |
| | |
| 21 | True |
| | |
| 22 | Null |
| | |
| 23 | Undefined value |
| | |
| 24..31 | (Reserved) |
| | |
| 32..255 | (Unassigned) |
+---------+-----------------+
Table 2: Simple Values
The 5-bit values of 25, 26, and 27 are for 16-bit, 32-bit, and 64-bit
IEEE 754 binary floating-point values. These floating-point values
are encoded in the additional bytes of the appropriate size. (See
Appendix D for some information about 16-bit floating point.)
2.4. Optional Tagging of Items
In CBOR, a data item can optionally be preceded by a tag to give it
additional semantics while retaining its structure. The tag is major
type 6, and represents an integer number as indicated by the tag's
integer value; the (sole) data item is carried as content data. If a
tag requires structured data, this structure is encoded into the
nested data item. The definition of a tag usually restricts what
kinds of nested data item or items can be carried by a tag.
The initial bytes of the tag follow the rules for positive integers
(major type 0). The tag is followed by a single data item of any
type. For example, assume that a byte string of length 12 is marked
with a tag to indicate it is a positive bignum (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.
Decoders do not need to understand tags, and thus tags may be of
little value in applications where the implementation creating a
particular CBOR data item and the implementation decoding that stream
know the semantic meaning of each item in the data flow. Their
primary purpose in this specification is to define common data types
such as dates. A secondary purpose is to allow optional tagging when
the decoder is a generic CBOR decoder that might be able to benefit
from hints about the content of items. Understanding the semantic
tags is optional for a decoder; it can just jump over the initial
bytes of the tag and interpret the tagged data item itself.
A tag always applies to the item that 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.
IANA maintains a registry of tag values as described in Section 7.2.
Table 3 provides a list of initial values, with definitions in the
rest of this section.
+--------------+------------------+---------------------------------+
| Tag | Data Item | Semantics |
+--------------+------------------+---------------------------------+
| 0 | UTF-8 string | Standard date/time string; see |
| | | Section 2.4.1 |
| | | |
| 1 | multiple | Epoch-based date/time; see |
| | | Section 2.4.1 |
| | | |
| 2 | byte string | Positive bignum; see Section |
| | | 2.4.2 |
| | | |
| 3 | byte string | Negative bignum; see Section |
| | | 2.4.2 |
| | | |
| 4 | array | Decimal fraction; see Section |
| | | 2.4.3 |
| | | |
| 5 | array | Bigfloat; see Section 2.4.3 |
| | | |
| 6..20 | (Unassigned) | (Unassigned) |
| | | |
| 21 | multiple | Expected conversion to |
| | | base64url encoding; see |
| | | Section 2.4.4.2 |
| | | |
| 22 | multiple | Expected conversion to base64 |
| | | encoding; see Section 2.4.4.2 |
| | | |
| 23 | multiple | Expected conversion to base16 |
| | | encoding; see Section 2.4.4.2 |
| | | |
| 24 | byte string | Encoded CBOR data item; see |
| | | Section 2.4.4.1 |
| | | |
| 25..31 | (Unassigned) | (Unassigned) |
| | | |
| 32 | UTF-8 string | URI; see Section 2.4.4.3 |
| | | |
| 33 | UTF-8 string | base64url; see Section 2.4.4.3 |
| | | |
| 34 | UTF-8 string | base64; see Section 2.4.4.3 |
| | | |
| 35 | UTF-8 string | Regular expression; see |
| | | Section 2.4.4.3 |
| | | |
| 36 | UTF-8 string | MIME message; see Section |
| | | 2.4.4.3 |
| | | |
| 37..55798 | (Unassigned) | (Unassigned) |
| | | |
| 55799 | multiple | Self-describe CBOR; see |
| | | Section 2.4.5 |
| | | |
| 55800+ | (Unassigned) | (Unassigned) |
+--------------+------------------+---------------------------------+
Table 3: Values for Tags
2.4.1. Date and Time
Tag value 0 is for date/time strings that follow the standard format
described in [RFC3339], as refined by Section 3.3 of [RFC4287].
Tag value 1 is for numerical representation of seconds relative to
1970-01-01T00:00Z in UTC time. (For the non-negative values that 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.
2.4.2. Bignums
Bignums are integers that do not fit into the basic integer
representations provided by major types 0 and 1. They are encoded as
a byte string data item, which is interpreted as an unsigned integer
n in network byte order. For tag value 2, the value of the bignum is
n. For tag value 3, the value of the bignum is -1 - n. Decoders
that understand these tags MUST be able to decode bignums that have
leading zeroes.
For example, the number 18446744073709551616 (2**64) is represented
as 0b110_00010 (major type 6, tag 2), followed by 0b010_01001 (major
type 2, length 9), followed by 0x010000000000000000 (one byte 0x01
and eight bytes 0x00). In hexadecimal:
C2 -- Tag 2
29 -- Byte string of length 9
010000000000000000 -- Bytes content
2.4.3. Decimal Fractions and Bigfloats
Decimal fractions combine an integer mantissa with a base-10 scaling
factor. They are most useful if an application needs the exact
representation of a decimal fraction such as 1.1 because there is no
exact representation for many decimal fractions in binary floating
point.
Bigfloats combine an integer mantissa with a base-2 scaling factor.
They are binary floating-point values that can exceed the range or
the precision of the three IEEE 754 formats supported by CBOR
(Section 2.3). Bigfloats may also be used by constrained
applications that need some basic binary floating-point capability
without the need for supporting IEEE 754.
A decimal fraction or a bigfloat is represented as a tagged array
that contains exactly two integer numbers: an exponent e and a
mantissa m. Decimal fractions (tag 4) use base-10 exponents; the
value of a decimal fraction data item is m*(10**e). Bigfloats (tag
5) use base-2 exponents; the value of a bigfloat data item is
m*(2**e). The exponent e MUST be represented in an integer of major
type 0 or 1, while the mantissa also can be a bignum (Section 2.4.2).
An example of a decimal fraction is that the number 273.15 could be
represented as 0b110_00100 (major type of 6 for the tag, additional
information of 4 for the type of tag), followed by 0b100_00010 (major
type of 4 for the array, additional information of 2 for the length
of the array), followed by 0b001_00001 (major type of 1 for the first
integer, additional information of 1 for the value of -2), followed
by 0b000_11001 (major type of 0 for the second integer, additional
information of 25 for a two-byte value), followed by
0b0110101010110011 (27315 in two bytes). In hexadecimal:
C4 -- Tag 4
82 -- Array of length 2
21 -- -2
19 6ab3 -- 27315
An example of a bigfloat is that the number 1.5 could be represented
as 0b110_00101 (major type of 6 for the tag, additional information
of 5 for the type of tag), followed by 0b100_00010 (major type of 4
for the array, additional information of 2 for the length of the
array), followed by 0b001_00000 (major type of 1 for the first
integer, additional information of 0 for the value of -1), followed
by 0b000_00011 (major type of 0 for the second integer, additional
information of 3 for the value of 3). In hexadecimal:
C5 -- Tag 5
82 -- Array of length 2
20 -- -1
03 -- 3
Decimal fractions and bigfloats provide no representation of
Infinity, -Infinity, or NaN; if these are needed in place of a
decimal fraction or bigfloat, the IEEE 754 half-precision
representations from Section 2.3 can be used. For constrained
applications, where there is a choice between representing a specific
number as an integer and as a decimal fraction or bigfloat (such as
when the exponent is small and non-negative), there is a quality-of-
implementation expectation that the integer representation is used
directly.
2.4.4. Content Hints
The tags in this section are for content hints that might be used by
generic CBOR processors.
2.4.4.1. Encoded CBOR Data Item
Sometimes it is beneficial to carry an embedded CBOR data item that
is not meant to be decoded immediately at the time the enclosing data
item is being parsed. Tag 24 (CBOR data item) can be used to tag the
embedded byte string as a data item encoded in CBOR format.
2.4.4.2. Expected Later Encoding for CBOR-to-JSON Converters
Tags 21 to 23 indicate that a byte string might require a specific
encoding when interoperating with a text-based representation. These
tags are useful when an encoder knows that the byte string data it is
writing is likely to be later converted to a particular JSON-based
usage. That usage specifies that some strings are encoded as base64,
base64url, and so on. The encoder uses byte strings instead of doing
the encoding itself to reduce the message size, to reduce the code
size of the encoder, or both. The encoder does not know whether or
not the converter will be generic, and therefore wants to say what it
believes is the proper way to convert binary strings to JSON.
The data item tagged can be a byte string or any other data item. In
the latter case, the tag applies to all of the byte string data items
contained in the data item, except for those contained in a nested
data item tagged with an expected conversion.
These three tag types suggest conversions to three of the base data
encodings defined in [RFC4648]. For base64url encoding, 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.
2.4.4.3. Encoded Text
Some text strings hold data that have formats widely used on the
Internet, and sometimes those formats can be validated and presented
to the application in appropriate form by the decoder. There are
tags for some of these formats.
o Tag 32 is for URIs, as defined in [RFC3986];
o Tags 33 and 34 are for base64url- and base64-encoded text strings,
as defined in [RFC4648];
o Tag 35 is for regular expressions in Perl Compatible Regular
Expressions (PCRE) / JavaScript syntax [ECMA262].
o Tag 36 is for MIME messages (including all headers), as defined in
[RFC2045];
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.
2.4.5. Self-Describe CBOR
In many applications, it will be clear from the context that CBOR is
being employed for encoding a data item. For instance, a specific
protocol might specify the use of CBOR, or a media type is indicated
that specifies its use. However, there may be applications where
such context information is not available, such as when CBOR data is
stored in a file and disambiguating metadata is not in use. Here, it
may help to have some distinguishing characteristics for the data
itself.
Tag 55799 is defined for this purpose. It does not impart any
special semantics on the data item that follows; that is, the
semantics of a data item tagged with tag 55799 is exactly identical
to the semantics of the data item itself.
The serialization of this tag is 0xd9d9f7, which appears not to be in
use as a distinguishing mark for frequently used file types. In
particular, it is not a valid start of a Unicode text in any Unicode
encoding if followed by a valid CBOR data item.
For instance, a decoder might be able to parse both CBOR and JSON.
Such a decoder would need to mechanically distinguish the two
formats. An easy way for an encoder to help the decoder would be to
tag the entire CBOR item with tag 55799, the serialization of which
will never be found at the beginning of a JSON text.
3. Creating CBOR-Based Protocols
Data formats such as CBOR are often used in environments where there
is no format negotiation. A specific design goal of CBOR is to not
need any included or assumed schema: a decoder can take a CBOR item
and decode it with no other knowledge.
Of course, in real-world implementations, the encoder and the decoder
will have a shared view of what should be in a CBOR data item. For
example, an agreed-to format might be "the item is an array whose
first value is a UTF-8 string, 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".
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.
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.
3.1. CBOR in Streaming Applications
In a streaming application, a data stream may be composed of a
sequence of CBOR data items concatenated back-to-back. In such an
environment, the decoder immediately begins decoding a new data item
if data is found after the end of a previous data item.
Not all of the bytes making up a data item may be immediately
available to the decoder; some decoders will buffer additional data
until a complete data item can be presented to the application.
Other decoders can present partial information about a top-level data
item to an application, such as the nested data items that could
already be decoded, or even parts of a byte string that hasn't
completely arrived yet.
Note that some applications and protocols will not want to use
indefinite-length encoding. Using indefinite-length encoding allows
an encoder to not need to 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.
3.2. Generic Encoders and Decoders
A generic CBOR decoder can decode all well-formed CBOR data and
present them to an application. CBOR data 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).
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
[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 (Section 3.5).
Generic decoders provide ways to present well-formed CBOR values,
both valid and invalid, to an application. The diagnostic notation
(Section 6) may be used to present well-formed CBOR values to humans.
Generic encoders provide an application interface that allows the
application to specify any well-formed value, including simple values
and tags unknown to the encoder.
3.3. Syntax Errors
A decoder encountering a CBOR data item that is not well-formed
generally can choose to completely fail the decoding (issue an error
and/or stop processing altogether), substitute the problematic data
and data items using a decoder-specific convention that clearly
indicates there has been a problem, or take some other action.
3.3.1. Incomplete CBOR Data Items
The representation of a CBOR data item has a specific length,
determined by its initial bytes and by the structure of any data
items enclosed in the data items. If less data is available, this
can be treated as a syntax error. A decoder may also implement
incremental parsing, that is, decode the data item as far as it is
available and present the data found so far (such as in an event-
based interface), with the option of continuing the decoding once
further data is available.
Examples of incomplete data items include:
o A decoder expects a certain number of array or map entries but
instead encounters the end of the data.
o A decoder processes what it expects to be the last pair in a map
and comes to the end of the data.
o A decoder has just seen a tag and then encounters the end of the
data.
o A decoder has seen the beginning of an indefinite-length item but
encounters the end of the data before it sees the "break" stop
code.
3.3.2. Malformed Indefinite-Length Items
Examples of malformed indefinite-length data items include:
o Within an indefinite-length byte string or text, a decoder finds
an item that is not of the appropriate major type before it finds
the "break" stop code.
o Within an indefinite-length map, a decoder encounters the "break"
stop code immediately after reading a key (the value is missing).
Another error is finding a "break" stop code at a point in the data
where there is no immediately enclosing (unclosed) indefinite-length
item.
3.3.3. Unknown Additional Information Values
At the time of writing, some additional information values are
unassigned and reserved for future versions of this document (see
Section 5.2). Since the overall syntax for these additional
information values is not yet defined, a decoder that sees an
additional information value that it does not understand cannot
continue parsing.
3.4. Other Decoding Errors
A CBOR data item may be syntactically well-formed but present a
problem with interpreting the data encoded in it in the CBOR data
model. Generally speaking, a decoder that finds a data item with
such a problem might issue a warning, might stop processing
altogether, might handle the error and make the problematic value
available to the application as such, or take some other type of
action.
Such problems might include:
Duplicate keys in a map: Generic decoders (Section 3.2) make data
available to applications using the native CBOR data model. That
data model includes maps (key-value mappings with unique keys),
not multimaps (key-value mappings where multiple entries can have
the same key). Thus, a generic decoder that gets a CBOR map item
that has duplicate keys will decode to a map with only one
instance of that key, or it might stop processing altogether. On
the other hand, a "streaming decoder" may not even be able to
notice (Section 3.7).
Inadmissible type on the value following a tag: Tags (Section 2.4)
specify what type of data item is supposed to follow the tag; for
example, the tags for positive or negative bignums are supposed to
be put on byte strings. A decoder that decodes the tagged data
item into a native representation (a native big integer in this
example) is expected to check the type of the data item being
tagged. Even decoders that don't have such native representations
available in their environment may perform the check on those tags
known to them and react appropriately.
Invalid UTF-8 string: A decoder might or might not want to verify
that the sequence of bytes in a UTF-8 string (major type 3) is
actually valid UTF-8 and react appropriately.
3.5. Handling Unknown Simple Values and Tags
A decoder that comes across a simple value (Section 2.3) that it does
not recognize, such as a value that was added to the IANA registry
after the decoder was deployed or a value that the decoder chose not
to implement, might issue a warning, might stop processing
altogether, might handle the error by making the unknown value
available to the application as such (as is expected of generic
decoders), or take some other type of action.
A decoder that comes across a tag (Section 2.4) that it does not
recognize, such as a tag that was added to the IANA registry after
the decoder was deployed or a tag that the decoder chose not to
implement, might issue a warning, might stop processing altogether,
might handle the error and present the unknown tag value together
with the contained data item to the application (as is expected of
generic decoders), might ignore the tag and simply present the
contained data item only to the application, or take some other type
of action.
3.6. Numbers
For the purposes of this specification, all number representations
for the same numeric value are equivalent. This means that an
encoder can encode a floating-point value of 0.0 as the integer 0.
It, however, also means that an application that expects to find
integer values only might find floating-point values if the encoder
decides these are desirable, such as when the floating-point value is
more compact than a 64-bit integer.
An application or protocol that uses CBOR might restrict the
representations of numbers. For instance, a protocol that only deals
with integers might say that floating-point numbers may not be used
and that decoders of that protocol do not need to be able to handle
floating-point numbers. Similarly, a protocol or application that
uses CBOR might say that decoders need to be able to handle either
type of number.
CBOR-based protocols should take into account that different language
environments pose different restrictions on the range and precision
of numbers that are representable. For example, the JavaScript
number system treats all numbers as floating point, which may result
in silent loss of precision in decoding integers with more than 53
significant bits. A protocol that uses numbers should define its
expectations on the handling of non-trivial numbers in decoders and
receiving applications.
A CBOR-based protocol that includes floating-point numbers can
restrict which of the three formats (half-precision, single-
precision, and double-precision) are to be supported. For an
integer-only application, a protocol may want to completely exclude
the use of floating-point values.
A CBOR-based protocol designed for compactness may want to exclude
specific integer encodings that are longer than necessary for the
application, such as to save the need to implement 64-bit integers.
There is an expectation that encoders will use the most compact
integer representation that can represent a given value. However, a
compact application should accept values that use a longer-than-
needed encoding (such as encoding "0" as 0b000_11101 followed by two
bytes of 0x00) as long as the application can decode an integer of
the given size.
3.7. Specifying Keys for Maps
The encoding and decoding applications need to agree on what types of
keys are going to be used in maps. In applications that need to
interwork with JSON-based applications, keys probably should be
limited to UTF-8 strings only; otherwise, there has to be a specified
mapping from the other CBOR types to Unicode characters, and this
often leads to implementation errors. In applications where keys are
numeric in nature and numeric ordering of keys is important to the
application, directly using the numbers for the keys is useful.
If multiple types of keys are to 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.
Decoders that deliver data items nested within a CBOR data item
immediately on decoding them ("streaming decoders") often do not keep
the state that is necessary to ascertain uniqueness of a key in a
map. Similarly, an encoder that can start encoding data items before
the enclosing data item is completely available ("streaming encoder")
may want to reduce its overhead significantly by relying on its data
source to maintain uniqueness.
A CBOR-based protocol 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
(Section 3.10).
The CBOR data model for maps does not allow ascribing semantics to
the order of the key/value pairs in the map representation.
Thus, it would be a very bad practice to define a CBOR-based protocol
in such a way that changing the key/value pair order in a map would
change the semantics, apart from trivial aspects (cache usage, etc.).
(A CBOR-based protocol can prescribe a specific order of
serialization, such as for canonicalization.)
Applications for constrained devices that have maps with 24 or fewer
frequently used keys should consider using small integers (and those
with up to 48 frequently used keys should consider also using small
negative integers) because the keys can then be encoded in a single
byte.
3.8. Undefined Values
In some CBOR-based protocols, the simple value (Section 2.3) of
Undefined might be used by an encoder as a substitute for a data item
with an encoding problem, in order to allow the rest of the enclosing
data items to be encoded without harm.
3.9. Canonical CBOR
Some protocols may want encoders to only emit CBOR in a particular
canonical format; those protocols might also have the decoders check
that their input is canonical. Those protocols are free to define
what they mean by a canonical format and what encoders and decoders
are expected to do. This section lists some suggestions for such
protocols.
If a protocol considers "canonical" to mean that two encoder
implementations starting with the same input data will produce the
same CBOR output, the following four rules would suffice:
o Integers must be as small as possible.
* 0 to 23 and -1 to -24 must be expressed in the same byte as the
major type;
* 24 to 255 and -25 to -256 must be expressed only with an
additional uint8_t;
* 256 to 65535 and -257 to -65536 must be expressed only with an
additional uint16_t;
* 65536 to 4294967295 and -65537 to -4294967296 must be expressed
only with an additional uint32_t.
o The expression of lengths in major types 2 through 5 must be as
short as possible. The rules for these lengths follow the above
rule for integers.
o The keys in every map must be sorted lowest value to highest.
Sorting is performed on the bytes of the representation of the key
data items without paying attention to the 3/5 bit splitting for
major types. (Note that this rule allows maps that have keys of
different types, even though that is probably a bad practice that
could lead to errors in some canonicalization implementations.)
The sorting rules are:
* If two keys have different lengths, the shorter one sorts
earlier;
* If two keys have the same length, the one with the lower value
in (byte-wise) lexical order sorts earlier.
o Indefinite-length items must be made into definite-length items.
If a protocol allows for IEEE floats, then additional
canonicalization rules might need to be added. One example rule
might be to have all floats start as a 64-bit float, then do a test
conversion to a 32-bit float; if the result is the same numeric
value, use the shorter value and repeat the process with a test
conversion to a 16-bit float. (This rule selects 16-bit float for
positive and negative Infinity as well.) Also, there are many
representations for NaN. If NaN is an allowed value, it must always
be represented as 0xf97e00.
CBOR tags present additional considerations for 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.
3.10. Strict Mode
Some areas of application of CBOR do not require canonicalization
(Section 3.9) but may require that different decoders reach the same
(semantically equivalent) results, even in the presence of
potentially malicious data. This can be required if one application
(such as a firewall or other protecting entity) makes a decision
based on the data that another application, which independently
decodes the data, relies on.
Normally, it is the responsibility of the sender to avoid ambiguously
decodable data. However, the sender might be an attacker specially
making up CBOR data such that it will be interpreted differently by
different decoders in an attempt to exploit that as a vulnerability.
Generic decoders used in applications where this might be a problem
need to support a strict mode in which it is also the responsibility
of the receiver to reject ambiguously decodable data. It is expected
that firewalls and other security systems that decode CBOR will only
decode in strict mode.
A decoder in strict mode will reliably reject any data that could be
interpreted by other decoders in different ways. It will reliably
reject data items with syntax errors (Section 3.3). It will also
expend the effort to reliably detect other decoding errors
(Section 3.4). In particular, a strict decoder needs to have an API
that reports an error (and does not return data) for a CBOR data item
that contains any of the following:
o a map (major type 5) that has more than one entry with the same
key
o a tag that is used on a data item of the incorrect type
o a data item that is incorrectly formatted for the type given to
it, such as invalid UTF-8 or data that cannot be interpreted with
the specific tag that it has been tagged with
A decoder in strict mode can do one of two things when it encounters
a tag or simple value that it does not recognize:
o It can report an error (and not return data).
o It can emit the unknown item (type, value, and, for tags, the
decoded tagged data item) to the application calling the decoder
with an indication that the decoder did not recognize that tag or
simple value.
The latter approach, which is also appropriate for non-strict
decoders, supports forward compatibility with newly registered tags
and simple values without the requirement to update the encoder at
the same time as the calling application. (For this, the API for the
decoder needs to have a way to mark unknown items so that the calling
application can handle them in a manner appropriate for the program.)
Since some of this processing may have an appreciable cost (in
particular with duplicate detection for maps), support of strict mode
is not a requirement placed on all CBOR decoders.
Some encoders will rely on their applications to provide input data
in such a way that unambiguously decodable CBOR results. A generic
encoder also may want to provide a strict mode where it reliably
limits its output to unambiguously decodable CBOR, independent of
whether or not its application is providing API-conformant data.
4. Converting Data between CBOR and JSON
This section gives non-normative advice about converting between CBOR
and JSON. Implementations of converters are free to use whichever
advice here they want.
It is worth noting that a JSON text is a sequence of characters, not
an encoded sequence of bytes, while a CBOR data item consists of
bytes, not characters.
4.1. Converting from CBOR to JSON
Most of the types in CBOR have direct analogs in JSON. However, some
do not, and someone implementing a CBOR-to-JSON converter has to
consider what to do in those cases. The following non-normative
advice deals with these by converting them to a single substitute
value, such as a JSON null.
o An integer (major type 0 or 1) becomes a JSON number.
o A byte string (major type 2) that is not embedded in a tag that
specifies a proposed encoding is encoded in base64url without
padding and becomes a JSON string.
o A UTF-8 string (major type 3) becomes a JSON string. Note that
JSON requires escaping certain characters (RFC 4627, Section 2.5):
quotation mark (U+0022), reverse solidus (U+005C), and the "C0
control characters" (U+0000 through U+001F). All other characters
are copied unchanged into the JSON UTF-8 string.
o An array (major type 4) becomes a JSON array.
o A map (major type 5) becomes a JSON object. This is possible
directly only if all keys are UTF-8 strings. A converter might
also convert other keys into UTF-8 strings (such as by converting
integers into strings containing their decimal representation);
however, doing so introduces a danger of key collision.
o False (major type 7, additional information 20) becomes a JSON
false.
o True (major type 7, additional information 21) becomes a JSON
true.
o Null (major type 7, additional information 22) becomes a JSON
null.
o A floating-point value (major type 7, additional information 25
through 27) becomes a JSON number if it is finite (that is, it can
be represented in a JSON number); if the value is non-finite (NaN,
or positive or negative Infinity), it is represented by the
substitute value.
o Any other simple value (major type 7, any additional information
value not yet discussed) is represented by the substitute value.
o A bignum (major type 6, tag value 2 or 3) is represented by
encoding its byte string in base64url without padding and becomes
a JSON string. For tag value 3 (negative bignum), a "~" (ASCII
tilde) is inserted before the base-encoded value. (The conversion
to a binary blob instead of a number is to prevent a likely
numeric overflow for the JSON decoder.)
o A byte string with an encoding hint (major type 6, tag value 21
through 23) is encoded as described and becomes a JSON string.
o For all other tags (major type 6, any other tag value), the
embedded CBOR item is represented as a JSON value; the tag value
is ignored.
o Indefinite-length items are made definite before conversion.
4.2. Converting from JSON to CBOR
All JSON values, once decoded, directly map into one or more CBOR
values. As with any kind of CBOR generation, decisions have to be
made with respect to number representation. In a suggested
conversion:
o JSON numbers without fractional parts (integer numbers) are
represented as integers (major types 0 and 1, possibly major type
6 tag value 2 and 3), choosing the shortest form; integers longer
than an implementation-defined threshold (which is usually either
32 or 64 bits) may instead be represented as floating-point
values. (If the JSON was generated from a JavaScript
implementation, its precision is already limited to 53 bits
maximum.)
o Numbers with fractional parts are represented as floating-point
values. Preferably, the shortest exact floating-point
representation is used; for instance, 1.5 is represented in a
16-bit floating-point value (not all implementations will be
capable of efficiently finding the minimum form, though). There
may be an implementation-defined limit to the precision that will
affect the precision of the represented values. Decimal
representation should only be used if that is specified in a
protocol.
CBOR has been designed to generally provide a more compact encoding
than JSON. One implementation strategy that might come to mind is to
perform a JSON-to-CBOR encoding in place in a single buffer. This
strategy would need to carefully consider a number of pathological
cases, such as that some strings represented with no or very few
escapes and longer (or much longer) than 255 bytes may expand when
encoded as UTF-8 strings in CBOR. Similarly, a few of the binary
floating-point representations might cause expansion from some short
decimal representations (1.1, 1e9) in JSON. This may be hard to get
right, and any ensuing vulnerabilities may be exploited by an
attacker.
5. Future Evolution of CBOR
Successful protocols evolve over time. New ideas appear,
implementation platforms improve, related protocols are developed and
evolve, and new requirements from applications and protocols are
added. Facilitating protocol evolution is therefore an important
design consideration for any protocol development.
For protocols that will use CBOR, CBOR provides some useful
mechanisms to facilitate their evolution. Best practices for this
are well known, particularly from JSON format development of JSON-
based protocols. Therefore, such best practices are outside the
scope of this specification.
However, facilitating the evolution of CBOR itself is very well
within its scope. CBOR is designed to both provide a stable basis
for development of CBOR-based protocols and to be able to evolve.
Since a successful protocol may live for decades, CBOR needs to be
designed for decades of use and evolution. This section provides
some guidance for the evolution of CBOR. It is necessarily more
subjective than other parts of this document. It is also necessarily
incomplete, lest it turn into a textbook on protocol development.
5.1. Extension Points
In a protocol design, opportunities for evolution are often included
in the form of extension points. For example, there may be a
codepoint space that is not fully allocated from the outset, and the
protocol is designed to tolerate and embrace implementations that
start using more codepoints than initially allocated.
Sizing the codepoint space may be difficult because the range
required may be hard to predict. An attempt should be made to make
the codepoint space large enough so that it can slowly be filled over
the intended lifetime of the protocol.
CBOR has three major extension points:
o the "simple" space (values in major type 7). Of the 24 efficient
(and 224 slightly less efficient) values, only a small number have
been allocated. Implementations receiving an unknown simple data
item may be able to process it as such, given that the structure
of the value is indeed simple. The IANA registry in Section 7.1
is the appropriate way to address the extensibility of this
codepoint space.
o the "tag" space (values in major type 6). Again, only a small
part of the codepoint space has been allocated, and the space is
abundant (although the early numbers are more efficient than the
later ones). Implementations receiving an unknown tag can choose
to simply ignore it or to process it as an unknown tag wrapping
the following data item. The IANA registry in Section 7.2 is the
appropriate way to address the extensibility of this codepoint
space.
o the "additional information" space. An implementation receiving
an unknown additional information value has no way to continue
parsing, so allocating codepoints to this space is a major step.
There are also very few codepoints left.
5.2. Curating the Additional Information Space
The human mind is sometimes drawn to filling in little perceived gaps
to make something neat. We expect the remaining gaps in the
codepoint space for the additional information values to be an
attractor for new ideas, just because they are there.
The present specification does not manage the additional information
codepoint space by an IANA registry. Instead, allocations out of
this space can only be done by updating this specification.
For an additional information value of n >= 24, the size of the
additional data typically is 2**(n-24) bytes. Therefore, additional
information values 28 and 29 should be viewed as candidates for
128-bit and 256-bit quantities, in case a need arises to add them to
the protocol. Additional information value 30 is then the only
additional information value available for general allocation, and
there should be a very good reason for allocating it before assigning
it through an update of this protocol.
6. Diagnostic Notation
CBOR is a binary interchange format. To facilitate documentation and
debugging, and in particular to facilitate communication between
entities cooperating in debugging, this section defines a simple
human-readable diagnostic notation. All actual interchange always
happens in the binary format.
Note that this truly is a diagnostic format; it is not meant to be
parsed. Therefore, no formal definition (as in ABNF) is given in
this document. (Implementers looking for a text-based format for
representing CBOR data items in configuration files may also want to
consider YAML [YAML].)
The diagnostic notation is loosely based on JSON as it is defined in
RFC 4627, extending it where needed.
The notation borrows the JSON syntax for numbers (integer and
floating point), True (>true<), False (>false<), Null (>null<), UTF-8
strings, arrays, and maps (maps are called objects in JSON; the
diagnostic notation extends JSON here by allowing any data item in
the key position). Undefined is written >undefined< as in
JavaScript. The non-finite floating-point numbers Infinity,
-Infinity, and NaN are written exactly as in this sentence (this is
also a way they can be written in JavaScript, although JSON does not
allow them). A tagged item is written as an integer number for the
tag followed by the item in parentheses; for instance, an RFC 3339
(ISO 8601) date could be notated as:
0("2013-03-21T20:04:00Z")
or the equivalent relative time as
1(1363896240)
Byte strings are notated in one of the base encodings, without
padding, enclosed in single quotes, prefixed by >h< for base16, >b32<
for base32, >h32< for base32hex, >b64< for base64 or base64url (the
actual encodings do not overlap, so the string remains unambiguous).
For example, the byte string 0x12345678 could be written h'12345678',
b32'CI2FM6A', or b64'EjRWeA'.
Unassigned simple values are given as "simple()" with the appropriate
integer in the parentheses. For example, "simple(42)" indicates
major type 7, value 42.
6.1. Encoding Indicators
Sometimes it is useful to indicate in the diagnostic notation which
of several alternative representations were actually used; for
example, a data item written >1.5< by a diagnostic decoder might have
been encoded as a half-, single-, or double-precision float.
The convention for encoding indicators is that anything starting with
an underscore and all following characters that are alphanumeric or
underscore, is an encoding indicator, and can be ignored by anyone
not interested in this information. Encoding indicators are always
optional.
A single underscore can be written after the opening brace of a map
or the opening bracket of an array to indicate that the data item was
represented in indefinite-length format. For example, [_ 1, 2]
contains an indicator that an indefinite-length representation was
used to represent the data item [1, 2].
An underscore followed by a decimal digit n indicates that the
preceding item (or, for arrays and maps, the item starting with the
preceding bracket or brace) was encoded with an additional
information value of 24+n. For example, 1.5_1 is a half-precision
floating-point number, while 1.5_3 is encoded as double precision.
This encoding indicator is not shown in Appendix A. (Note that the
encoding indicator "_" is thus an abbreviation of the full form "_7",
which is not used.)
As a special case, byte and text strings of indefinite length can be
notated in the form (_ h'0123', h'4567') and (_ "foo", "bar").
7. IANA Considerations
IANA has created two registries for new CBOR values. The registries
are separate, that is, not under an umbrella registry, and follow the
rules in [RFC5226]. IANA has also assigned a new MIME media type and
an associated Constrained Application Protocol (CoAP) Content-Format
entry.
7.1. Simple Values Registry
IANA has created the "Concise Binary Object Representation (CBOR)
Simple Values" registry. The initial values are shown in Table 2.
New entries in the range 0 to 19 are assigned by Standards Action.
It is suggested that these Standards Actions allocate values starting
with the number 16 in order to reserve the lower numbers for
contiguous blocks (if any).
New entries in the range 32 to 255 are assigned by Specification
Required.
7.2. Tags Registry
IANA has created the "Concise Binary Object Representation (CBOR)
Tags" registry. The initial values are shown in Table 3.
New entries in the range 0 to 23 are assigned by Standards Action.
New entries in the range 24 to 255 are assigned by Specification
Required. New entries in the range 256 to 18446744073709551615 are
assigned by First Come First Served. The template for registration
requests is:
o Data item
o Semantics (short form)
In addition, First Come First Served requests should include:
o Point of contact
o Description of semantics (URL)
This description is optional; the URL can point to something like
an Internet-Draft or a web page.
7.3. Media Type ("MIME Type")
The Internet media type [RFC6838] for CBOR data is application/cbor.
Type name: application
Subtype name: cbor
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: binary
Security considerations: See Section 8 of this document
Interoperability considerations: n/a
Published specification: This document
Applications that use this media type: None yet, but it is expected
that this format will be deployed in protocols and applications.
Additional information:
Magic number(s): n/a
File extension(s): .cbor
Macintosh file type code(s): n/a
Person & email address to contact for further information:
Carsten Bormann
cabo@tzi.org
Intended usage: COMMON
Restrictions on usage: none
Author:
Carsten Bormann <cabo@tzi.org>
Change controller:
The IESG <iesg@ietf.org>
7.4. CoAP Content-Format
Media Type: application/cbor
Encoding: -
Id: 60
Reference: [RFC7049]
7.5. The +cbor Structured Syntax Suffix Registration
Name: Concise Binary Object Representation (CBOR)
+suffix: +cbor
References: [RFC7049]
Encoding Considerations: CBOR is a binary format.
Interoperability Considerations: n/a
Fragment Identifier Considerations:
The syntax and semantics of fragment identifiers specified for
+cbor SHOULD be as specified for "application/cbor". (At
publication of this document, there is no fragment identification
syntax defined for "application/cbor".)
The syntax and semantics for fragment identifiers for a specific
"xxx/yyy+cbor" SHOULD be processed as follows:
For cases defined in +cbor, where the fragment identifier resolves
per the +cbor rules, then process as specified in +cbor.
For cases defined in +cbor, where the fragment identifier does not
resolve per the +cbor rules, then process as specified in
"xxx/yyy+cbor".
For cases not defined in +cbor, then process as specified in
"xxx/yyy+cbor".
Security Considerations: See Section 8 of this document
Contact:
Apps Area Working Group (apps-discuss@ietf.org)
Author/Change Controller:
The Apps Area Working Group.
The IESG has change control over this registration.
8. Security Considerations
A network-facing application can exhibit vulnerabilities in its
processing logic for incoming data. Complex parsers are well known
as a likely source of such vulnerabilities, such as the ability to
remotely crash a node, or even remotely execute arbitrary code on it.
CBOR attempts to narrow the opportunities for introducing such
vulnerabilities by reducing parser complexity, by giving the entire
range of encodable values a meaning where possible.
Resource exhaustion attacks might attempt to lure a decoder into
allocating very big data items (strings, arrays, maps) or exhaust the
stack depth by setting up deeply nested items. Decoders need to have
appropriate resource management to mitigate these attacks. (Items
for which very large sizes are given can also attempt to exploit
integer overflow vulnerabilities.)
Applications where 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 (Section 3.10).
9. Acknowledgements
CBOR was inspired by MessagePack. MessagePack was developed and
promoted by Sadayuki Furuhashi ("frsyuki"). This reference to
MessagePack is solely for attribution; CBOR is not intended as a
version of or replacement for MessagePack, as it has different design
goals and requirements.
The need for functionality beyond the original MessagePack
Specification became obvious to many people at about the same time
around the year 2012. BinaryPack is a minor derivation of
MessagePack that was developed by Eric Zhang for the binaryjs
project. A similar, but different, extension was made by Tim Caswell
for his msgpack-js and msgpack-js-browser projects. Many people have
contributed to the recent discussion about extending MessagePack to
separate text string representation from byte string representation.
The encoding of the additional information in CBOR was inspired by
the encoding of length information designed by Klaus Hartke for CoAP.
This document also incorporates suggestions made by many people,
notably Dan Frost, James Manger, Joe Hildebrand, Keith Moore, Matthew
Lepinski, Nico Williams, Phillip Hallam-Baker, Ray Polk, Tim Bray,
Tony Finch, Tony Hansen, and Yaron Sheffer.
10. References
10.1. Normative References
[ECMA262] European Computer Manufacturers Association, "ECMAScript
Language Specification 5.1 Edition", ECMA Standard
ECMA-262, June 2011, <http://www.ecma-international.org/
publications/files/ecma-st/ECMA-262.pdf>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, December 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[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>.
10.2. Informative References
[ASN.1] International Telecommunication Union, "Information
Technology -- ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, 1994.
[BSON] Various, "BSON - Binary JSON", 2013,
<http://bsonspec.org/>.
[CNN-TERMS]
Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained Node Networks", Work in Progress, July 2013.
[MessagePack]
Furuhashi, S., "MessagePack", 2013, <http://msgpack.org/>.
[RFC0713] Haverty, J., "MSDTP-Message Services Data Transmission
Protocol", RFC 713, April 1976.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, January 2013.
[UBJSON] The Buzz Media, "Universal Binary JSON Specification",
2013, <http://ubjson.org/>.
[YAML] Ben-Kiki, O., Evans, C., and I. Net, "YAML Ain't Markup
Language (YAML[TM]) Version 1.2", 3rd Edition, October
2009, <http://www.yaml.org/spec/1.2/spec.html>.
Appendix A. Examples
The following table provides some CBOR-encoded values in hexadecimal
(right column), together with diagnostic notation for these values
(left column). Note that the string "\u00fc" is one form of
diagnostic notation for a UTF-8 string containing the single Unicode
character U+00FC, LATIN SMALL LETTER U WITH DIAERESIS (u umlaut).
Similarly, "\u6c34" is a UTF-8 string in diagnostic notation with a
single character U+6C34 (CJK UNIFIED IDEOGRAPH-6C34, often
representing "water"), and "\ud800\udd51" is a UTF-8 string in
diagnostic notation with a single character U+10151 (GREEK ACROPHONIC
ATTIC FIFTY STATERS). (Note that all these single-character strings
could also be represented in native UTF-8 in diagnostic notation,
just not in an ASCII-only specification like the present one.) In
the diagnostic notation provided for bignums, their intended numeric
value is shown as a decimal number (such as 18446744073709551616)
instead of showing a tagged byte string (such as
2(h'010000000000000000')).
+------------------------------+------------------------------------+
| Diagnostic | Encoded |
+------------------------------+------------------------------------+
| 0 | 0x00 |
| | |
| 1 | 0x01 |
| | |
| 10 | 0x0a |
| | |
| 23 | 0x17 |
| | |
| 24 | 0x1818 |
| | |
| 25 | 0x1819 |
| | |
| 100 | 0x1864 |
| | |
| 1000 | 0x1903e8 |
| | |
| 1000000 | 0x1a000f4240 |
| | |
| 1000000000000 | 0x1b000000e8d4a51000 |
| | |
| 18446744073709551615 | 0x1bffffffffffffffff |
| | |
| 18446744073709551616 | 0xc249010000000000000000 |
| | |
| -18446744073709551616 | 0x3bffffffffffffffff |
| | |
| -18446744073709551617 | 0xc349010000000000000000 |
| | |
| -1 | 0x20 |
| | |
| -10 | 0x29 |
| | |
| -100 | 0x3863 |
| | |
| -1000 | 0x3903e7 |
| | |
| 0.0 | 0xf90000 |
| | |
| -0.0 | 0xf98000 |
| | |
| 1.0 | 0xf93c00 |
| | |
| 1.1 | 0xfb3ff199999999999a |
| | |
| 1.5 | 0xf93e00 |
| | |
| 65504.0 | 0xf97bff |
| | |
| 100000.0 | 0xfa47c35000 |
| | |
| 3.4028234663852886e+38 | 0xfa7f7fffff |
| | |
| 1.0e+300 | 0xfb7e37e43c8800759c |
| | |
| 5.960464477539063e-8 | 0xf90001 |
| | |
| 0.00006103515625 | 0xf90400 |
| | |
| -4.0 | 0xf9c400 |
| | |
| -4.1 | 0xfbc010666666666666 |
| | |
| Infinity | 0xf97c00 |
| | |
| NaN | 0xf97e00 |
| | |
| -Infinity | 0xf9fc00 |
| | |
| Infinity | 0xfa7f800000 |
| | |
| NaN | 0xfa7fc00000 |
| | |
| -Infinity | 0xfaff800000 |
| | |
| Infinity | 0xfb7ff0000000000000 |
| | |
| NaN | 0xfb7ff8000000000000 |
| | |
| -Infinity | 0xfbfff0000000000000 |
| | |
| false | 0xf4 |
| | |
| true | 0xf5 |
| | |
| null | 0xf6 |
| | |
| undefined | 0xf7 |
| | |
| simple(16) | 0xf0 |
| | |
| simple(24) | 0xf818 |
| | |
| simple(255) | 0xf8ff |
| | |
| 0("2013-03-21T20:04:00Z") | 0xc074323031332d30332d32315432303a |
| | 30343a30305a |
| | |
| 1(1363896240) | 0xc11a514b67b0 |
| | |
| 1(1363896240.5) | 0xc1fb41d452d9ec200000 |
| | |
| 23(h'01020304') | 0xd74401020304 |
| | |
| 24(h'6449455446') | 0xd818456449455446 |
| | |
| 32("http://www.example.com") | 0xd82076687474703a2f2f7777772e6578 |
| | 616d706c652e636f6d |
| | |
| h'' | 0x40 |
| | |
| h'01020304' | 0x4401020304 |
| | |
| "" | 0x60 |
| | |
| "a" | 0x6161 |
| | |
| "IETF" | 0x6449455446 |
| | |
| "\"\\" | 0x62225c |
| | |
| "\u00fc" | 0x62c3bc |
| | |
| "\u6c34" | 0x63e6b0b4 |
| | |
| "\ud800\udd51" | 0x64f0908591 |
| | |
| [] | 0x80 |
| | |
| [1, 2, 3] | 0x83010203 |
| | |
| [1, [2, 3], [4, 5]] | 0x8301820203820405 |
| | |
| [1, 2, 3, 4, 5, 6, 7, 8, 9, | 0x98190102030405060708090a0b0c0d0e |
| 10, 11, 12, 13, 14, 15, 16, | 0f101112131415161718181819 |
| 17, 18, 19, 20, 21, 22, 23, | |
| 24, 25] | |
| | |
| {} | 0xa0 |
| | |
| {1: 2, 3: 4} | 0xa201020304 |
| | |
| {"a": 1, "b": [2, 3]} | 0xa26161016162820203 |
| | |
| ["a", {"b": "c"}] | 0x826161a161626163 |
| | |
| {"a": "A", "b": "B", "c": | 0xa5616161416162614261636143616461 |
| "C", "d": "D", "e": "E"} | 4461656145 |
| | |
| (_ h'0102', h'030405') | 0x5f42010243030405ff |
| | |
| (_ "strea", "ming") | 0x7f657374726561646d696e67ff |
| | |
| [_ ] | 0x9fff |
| | |
| [_ 1, [2, 3], [_ 4, 5]] | 0x9f018202039f0405ffff |
| | |
| [_ 1, [2, 3], [4, 5]] | 0x9f01820203820405ff |
| | |
| [1, [2, 3], [_ 4, 5]] | 0x83018202039f0405ff |
| | |
| [1, [_ 2, 3], [4, 5]] | 0x83019f0203ff820405 |
| | |
| [_ 1, 2, 3, 4, 5, 6, 7, 8, | 0x9f0102030405060708090a0b0c0d0e0f |
| 9, 10, 11, 12, 13, 14, 15, | 101112131415161718181819ff |
| 16, 17, 18, 19, 20, 21, 22, | |
| 23, 24, 25] | |
| | |
| {_ "a": 1, "b": [_ 2, 3]} | 0xbf61610161629f0203ffff |
| | |
| ["a", {_ "b": "c"}] | 0x826161bf61626163ff |
| | |
| {_ "Fun": true, "Amt": -2} | 0xbf6346756ef563416d7421ff |
+------------------------------+------------------------------------+
Table 4: Examples of Encoded CBOR Data Items
Appendix B. Jump Table
For brevity, this jump table does not show initial bytes that are
reserved for future extension. It also only shows a selection of the
initial bytes that can be used for optional features. (All unsigned
integers are in network byte order.)
+-----------------+-------------------------------------------------+
| Byte | Structure/Semantics |
+-----------------+-------------------------------------------------+
| 0x00..0x17 | Integer 0x00..0x17 (0..23) |
| | |
| 0x18 | Unsigned integer (one-byte uint8_t follows) |
| | |
| 0x19 | Unsigned integer (two-byte uint16_t follows) |
| | |
| 0x1a | Unsigned integer (four-byte uint32_t follows) |
| | |
| 0x1b | Unsigned integer (eight-byte uint64_t follows) |
| | |
| 0x20..0x37 | Negative integer -1-0x00..-1-0x17 (-1..-24) |
| | |
| 0x38 | Negative integer -1-n (one-byte uint8_t for n |
| | follows) |
| | |
| 0x39 | Negative integer -1-n (two-byte uint16_t for n |
| | follows) |
| | |
| 0x3a | Negative integer -1-n (four-byte uint32_t for n |
| | follows) |
| | |
| 0x3b | Negative integer -1-n (eight-byte uint64_t for |
| | n follows) |
| | |
| 0x40..0x57 | byte string (0x00..0x17 bytes follow) |
| | |
| 0x58 | byte string (one-byte uint8_t for n, and then n |
| | bytes follow) |
| | |
| 0x59 | byte string (two-byte uint16_t for n, and then |
| | n bytes follow) |
| | |
| 0x5a | byte string (four-byte uint32_t for n, and then |
| | n bytes follow) |
| | |
| 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 |
+-----------------+-------------------------------------------------+
Table 5: Jump Table for Initial Byte
Appendix C. Pseudocode
The well-formedness of a CBOR item can be checked by the pseudocode
in Figure 1. The data is well-formed if and only if:
o the pseudocode does not "fail";
o after execution of the pseudocode, no bytes are left in the input
(except in streaming applications)
The pseudocode has the following prerequisites:
o take(n) reads n bytes from the input data and returns them as a
byte string. If n bytes are no longer available, take(n) fails.
o uint() converts a byte string into an unsigned integer by
interpreting the byte string in network byte order.
o Arithmetic works as in C.
o All variables are unsigned integers of sufficient range.
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
}
Figure 1: Pseudocode for Well-Formedness Check
Note that the remaining complexity of a complete CBOR decoder is
about presenting data that has been parsed to the application in an
appropriate form.
Major types 0 and 1 are designed in such a way that they can be
encoded in C from a signed integer without actually doing an if-then-
else for positive/negative (Figure 2). This uses the fact that
(-1-n), the transformation for major type 1, is the same as ~n
(bitwise complement) in C unsigned arithmetic; ~n can then be
expressed as (-1)^n for the negative case, while 0^n leaves n
unchanged for non-negative. The sign of a number can be converted to
-1 for negative and 0 for non-negative (0 or positive) by arithmetic-
shifting the number by one bit less than the bit length of the number
(for example, by 63 for 64-bit numbers).
void encode_sint(int64_t n) {
uint64t ui = n >> 63; // extend sign to whole length
mt = ui & 0x20; // extract major type
ui ^= n; // complement negatives
if (ui < 24)
*p++ = mt + ui;
else if (ui < 256) {
*p++ = mt + 24;
*p++ = ui;
} else
...
Figure 2: Pseudocode for Encoding a Signed Integer
Appendix D. Half-Precision
As half-precision floating-point numbers were only added to IEEE 754
in 2008, today's programming platforms often still only have limited
support for them. It is very easy to include at least decoding
support for them even without such support. An example of a small
decoder for half-precision floating-point numbers in the C language
is shown in Figure 3. A similar program for Python is in Figure 4;
this code assumes that the 2-byte value has already been decoded as
an (unsigned short) integer in network byte order (as would be done
by the pseudocode in Appendix C).
#include <math.h>
double decode_half(unsigned char *halfp) {
int half = (halfp[0] << 8) + halfp[1];
int exp = (half >> 10) & 0x1f;
int mant = half & 0x3ff;
double val;
if (exp == 0) val = ldexp(mant, -24);
else if (exp != 31) val = ldexp(mant + 1024, exp - 25);
else val = mant == 0 ? INFINITY : NAN;
return half & 0x8000 ? -val : val;
}
Figure 3: C Code for a Half-Precision Decoder
import struct
from math import ldexp
def decode_single(single):
return struct.unpack("!f", struct.pack("!I", single))[0]
def decode_half(half):
valu = (half & 0x7fff) << 13 | (half & 0x8000) << 16
if ((half & 0x7c00) != 0x7c00):
return ldexp(decode_single(valu), 112)
return decode_single(valu | 0x7f800000)
Figure 4: Python Code for a Half-Precision Decoder
Appendix E. Comparison of Other Binary Formats to CBOR's Design
Objectives
The proposal for CBOR follows a history of binary formats that is as
long as the history of computers themselves. Different formats have
had different objectives. In most cases, the objectives of the
format were never stated, although they can sometimes be implied by
the context where the format was first used. Some formats were meant
to be universally usable, although history has proven that no binary
format meets the needs of all protocols and applications.
CBOR differs from many of these formats due to it starting with a set
of objectives and attempting to meet just those. This section
compares a few of the dozens of formats with CBOR's objectives in
order to help the reader decide if they want to use CBOR or a
different format for a particular protocol or application.
Note that the discussion here is not meant to be a criticism of any
format: to the best of our knowledge, no format before CBOR was meant
to cover CBOR's objectives in the priority we have assigned them. A
brief recap of the objectives from Section 1.1 is:
1. unambiguous encoding of most common data formats from Internet
standards
2. code compactness for encoder or decoder
3. no schema description needed
4. reasonably compact serialization
5. applicability to constrained and unconstrained applications
6. good JSON conversion
7. extensibility
E.1. ASN.1 DER, BER, and PER
[ASN.1] has many serializations. In the IETF, DER and BER are the
most common. The serialized output is not particularly compact for
many items, and the code needed to decode numeric items can be
complex on a constrained device.
Few (if any) IETF protocols have adopted one of the several variants
of Packed Encoding Rules (PER). There could be many reasons for
this, but one that is commonly stated is that PER makes use of the
schema even for parsing the surface structure of the data stream,
requiring significant tool support. There are different versions of
the ASN.1 schema language in use, which has also hampered adoption.
E.2. MessagePack
[MessagePack] is a concise, widely implemented counted binary
serialization format, similar in many properties to CBOR, although
somewhat less regular. While the data model can be used to represent
JSON data, MessagePack has also been used in many remote procedure
call (RPC) applications and for long-term storage of data.
MessagePack has been essentially stable since it was first published
around 2011; it has not yet had a transition. The evolution of
MessagePack is impeded by an imperative to maintain complete
backwards compatibility with existing stored data, while only few
bytecodes are still available for extension. Repeated requests over
the years from the MessagePack user community to separate out binary
and text strings in the encoding recently have led to an extension
proposal that would leave MessagePack's "raw" data ambiguous between
its usages for binary and text data. The extension mechanism for
MessagePack remains unclear.
E.3. BSON
[BSON] is a data format that was developed for the storage of JSON-
like maps (JSON objects) in the MongoDB database. Its major
distinguishing feature is the capability for in-place update,
foregoing a compact representation. BSON uses a counted
representation except for map keys, which are null-byte terminated.
While BSON can be used for the representation of JSON-like objects on
the wire, its specification is dominated by the requirements of the
database application and has become somewhat baroque. The status of
how BSON extensions will be implemented remains unclear.
E.4. UBJSON
[UBJSON] has a design goal to make JSON faster and somewhat smaller,
using a binary format that is limited to exactly the data model JSON
uses. Thus, there is expressly no intention to support, for example,
binary data; however, there is a "high-precision number", expressed
as a character string in JSON syntax. UBJSON is not optimized for
code compactness, and its type byte coding is optimized for human
recognition and not for compact representation of native types such
as small integers. Although UBJSON is mostly counted, it provides a
reserved "unknown-length" value to support streaming of arrays and
maps (JSON objects). Within these containers, UBJSON also has a
"Noop" type for padding.
E.5. MSDTP: RFC 713
Message Services Data Transmission (MSDTP) is a very early example of
a compact message format; it is described in [RFC0713], written in
1976. It is included here for its historical value, not because it
was ever widely used.
E.6. Conciseness on the Wire
While CBOR's design objective of code compactness for encoders and
decoders is a higher priority than its objective of conciseness on
the wire, many people focus on the wire size. Table 6 shows some
encoding examples for the simple nested array [1, [2, 3]]; where some
form of indefinite-length encoding is supported by the encoding,
[_ 1, [2, 3]] (indefinite length on the outer array) is also shown.
+---------------+-------------------------+-------------------------+
| Format | [1, [2, 3]] | [_ 1, [2, 3]] |
+---------------+-------------------------+-------------------------+
| RFC 713 | c2 05 81 c2 02 82 83 | |
| | | |
| ASN.1 BER | 30 0b 02 01 01 30 06 02 | 30 80 02 01 01 30 06 02 |
| | 01 02 02 01 03 | 01 02 02 01 03 00 00 |
| | | |
| MessagePack | 92 01 92 02 03 | |
| | | |
| BSON | 22 00 00 00 10 30 00 01 | |
| | 00 00 00 04 31 00 13 00 | |
| | 00 00 10 30 00 02 00 00 | |
| | 00 10 31 00 03 00 00 00 | |
| | 00 00 | |
| | | |
| UBJSON | 61 02 42 01 61 02 42 02 | 61 ff 42 01 61 02 42 02 |
| | 42 03 | 42 03 45 |
| | | |
| CBOR | 82 01 82 02 03 | 9f 01 82 02 03 ff |
+---------------+-------------------------+-------------------------+
Table 6: Examples for Different Levels of Conciseness
Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
EMail: cabo@tzi.org
Paul Hoffman
VPN Consortium
EMail: paul.hoffman@vpnc.org