# # Specification: DAG-CBOR

**Status: Descriptive - Draft**

DAG-CBOR supports the full IPLD Data Model.

DAG-CBOR uses the Concise Binary Object Representation (CBOR) (opens new window) data format, which natively supports all IPLD Data Model Kinds.

## # Format

The CBOR IPLD format is called DAG-CBOR to disambiguate it from regular CBOR. Most simple CBOR objects are valid DAG-CBOR. The primary differences are:

- tag
`42`

interpreted as CIDs, no other tags are supported - maps may only be keyed by strings
- additional strictness requirements are applied to ensure canonical data encoding forms

## # Links

As with all IPLD formats, DAG-CBOR must be able to encode Links. In DAG-CBOR, links are the binary form of a [CID] encoded using the raw-binary identity Multibase (opens new window). That is, the Multibase identity prefix (`0x00`

) is prepended to the binary form of a CID and this new byte array is encoded into CBOR as a byte-string (major type 2), with the tag `42`

.

The inclusion of the Multibase prefix exists for historical reasons and the identity prefix *must not* be omitted.

## # Map Keys

In DAG-CBOR, map keys must be strings, as defined by the IPLD Data Model. Other map keys, such as ints, are not supported and should be rejected when encountered.

## # Strictness

DAG-CBOR requires that there exist a single, canonical way of encoding any given object, and that encoded forms contain no superfluous data that may be ignored or lost in a round-trip decode/encode.

Therefore the DAG-CBOR codec must:

- Use no tags other than the CID tag (
`42`

). A valid DAG-CBOR encoder must not encode using any additional tags and a valid DAG-CBOR decoder must reject objects containing additional tags as invalid.- This includes any of the initial values of the tag registry in section 2.4 of the CBOR specification (opens new window), such as dates, bignums, bigfloats, URIs, regular expressions and other complex, or simple values whether or not they map to the IPLD Data Model.

- The only usable major type 7 minor types are those for encoding Floats (
`25`

,`26`

,`27`

), True (`20`

), False (`21`

) and Null (`22`

).- "Simple values" are not supported. This includes all registered or unregistered simple values that are encoded with a major type 7.
- Undefined (
`23`

) is not supported.

- Use the canonical CBOR encoding defined by the suggestions in section 3.9 of the CBOR specification (opens new window). A valid DAG-CBOR decoder should reject objects not following these restrictions as invalid. Specifically:
- Integer encoding must be as short as possible.
- The expression of lengths in major types 2 through 5 must be as short as possible.
- The expression of tag numbers (specifically only
`42`

) must be as short as possible for major type 6. Therefore, for valid DAG-CBOR, the only tag token that can appear is`0xd82a`

- where`0xd8`

is "major type 6 with 8-bit integer to follow" and`0x2a`

is the number`42`

. - The keys in every map must be sorted lowest value to highest. Sorting is performed on the bytes of the representation of the keys.
- 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.

- Indefinite-length items are not supported, only definite-length items are usable. This includes strings, bytes, lists and maps. The "break" token is also not supported.

- Encode and decode a single top-level CBOR object and not allow back-to-back concatenated objects, as suggested by section 3.1 of the CBOR specification (opens new window) for
*streaming applications*. All bytes of an encoded DAG-CBOR object must decode to a single object. Extraneous bytes, whether valid or invalid CBOR, should fail validation. - Floating point values are always encoded in 64-bit, double-precision form, regardless of whether they can be represented as half (16) or single (32) precision.
- IEEE 754 special values
`NaN`

,`Infinity`

and`-Infinity`

should not be accepted as they do not appear in the IPLD Data Model. Therefore, tokens`0xf97c00`

(`Infinity`

),`0xf97e00`

(`NaN`

) and`0xf9fc00`

(`-Infinity`

) and their 32-bit and 64-bit variants, should not appear, or be accepted in DAG-CBOR binary form.

## # Implementations

### # JavaScript

dag-cbor (opens new window), used by ipld (opens new window) and @ipld/block (opens new window) adheres to this specification, with the following caveats:

- Strictness is not yet enforced on decode, blocks encoded that don't follow the strictness rules are not rejected
- Floating point values are encoded as their smallest form rather than always double-precision.
- Many additional object types outside of the Data Model are currently accepted for encoding.
- IEEE 754 special values
`NaN`

,`Infinity`

and`-Infinity`

are accepted for decode and encode.

### # Go

ipld-cbor (opens new window) and ipld-prime (opens new window) adhere to this specification, with the following caveats:

- Strictness is not yet enforced on decode, blocks encoded that don't follow the strictness rules are not rejected
- IEEE 754 special values
`NaN`

,`Infinity`

and`-Infinity`

are accepted for decode and encode.

### # Java

java ipld from Peergos (opens new window) adhere to this specification, with the following caveats:

- Strictness is not yet enforced on decode, blocks encoded that don't follow the strictness rules are not rejected
- Floats are disabled

## # Limitations

### # JavaScript

Users of DAG-CBOR that expect their data may be consumed or produced by JavaScript at some point should be aware of limitations that the language imposes on its use of DAG-CBOR, specifically concerning numbers.

All JavaScript numbers, both floating point and integer, (using the `Number`

(opens new window) primitive wrapper) are represented internally as 64-bit IEEE 754 (opens new window) floating-point values (i.e. double-precision). Some implications within JavaScript of this design choice are:

- There is no clear differentiation between a pure integer type and a floating-point number where a developer may wish to have such a differentiation.
- By convention, JavaScript engines and developers usually omit the decimal point when representing whole numbers, simulating integers where the number is not actually stored as an integer.
- There are limits on maximum and minimum safe integer sizes representable in JavaScript that are more constrained than those of languages where there are 64-bit integer types. Numbers outside of the range of
`Number.MAX_SAFE_INTEGER`

(`2`

^{53}`- 1`

) and`Number.MIN_SAFE_INTEGER`

(`-(2`

^{53}`- 1)`

) cannot be safely manipulated or inspected as they incur rounding effects imposed by the IEEE 754 representation. - Native bit-wise operations on "integers" are not able to be performed outside of the 32-bit range; larger numbers will be truncated.

The current CBOR encoder/decoder used by the primary JavaScript DAG-CBOR implementation uses the bignumber.js (opens new window) library to handle large numbers in some cases, although reliance on its wrapper type is not recommended by DAG-CBOR users.

The implications for DAG-CBOR of these limitaitons are:

- Any
`Number`

serialized by the JavaScript CBOR encoder relies on a whole-number check (e.g.`x % 1 === 0`

) to determine whether it should be encoded as an integer or a float. - Any float deserialized by the JavaScript CBOR decoder that does not have a fractional component will be indistinguishable from an integer to a JavaScript program.
- Any
`Number`

greater than`Number.MAX_SAFE_INTEGER`

or less than`Number.MIN_SAFE_INTEGER`

cannot be properly inspected for its whole-number status and is therefore encoded by the JavaScript CBOR encoder as float regardless of whether it is a whole-number or has a fractional component. - Any integer deserialized by the JavaScript CBOR decoder greater than
`Number.MAX_SAFE_INTEGER`

or less than`Number.MIN_SAFE_INTEGER`

will be returned as a bignumber.js wrapper type, which may be unexpected to users and have unexpected effects on downstream code.

A new BigInt (opens new window) built-in type is currently being adopted across JavaScript engines. Once support is widely available, it is expected that this type will assist with some of these challenges.