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DAG-PB Specification
Spec

DAG-PB Spec

Status: Descriptive - Final

DAG-PB is an IPLD codec that uses Protocol Buffers to describe a binary format that can encode a byte array and an associated list of links. It is the primary means of encoding structured file data for IPFS, serving as the encoded data carrier for UnixFS.

DAG-PB does not support the full IPLD Data Model.

Implementations

Serial Format

The DAG-PB IPLD serial format is described with a single protobuf:

message PBLink {
  // binary CID (with no multibase prefix) of the target object
  optional bytes Hash = 1;

  // UTF-8 string name
  optional string Name = 2;

  // cumulative size of target object
  optional uint64 Tsize = 3;
}

message PBNode {
  // refs to other objects
  repeated PBLink Links = 2;

  // opaque user data
  optional bytes Data = 1;
}

Protobuf Strictness

DAG-PB aims to have a canonical form for any given set of data. Therefore, in addition to the standard Protobuf parsing rules, DAG-PB decoders should enforce additional constraints to ensure canonical forms (where possible):

  1. Fields in the PBLink message must appear in the order as defined by the Protobuf schema above, following the field numbers. Blocks with out-of-order PBLink fields should be rejected. (Note that it is common for Protobuf decoders to accept out-of-order field entries, which means the DAG-PB spec is somewhat stricter than may be seen as typical for other Protobuf-based formats.)
  2. Fields in the PBNode message must be encoded in the order as defined by the Protobuf schema above. Note that this order does not follow the field numbers. The decoder should accept either order, as IPFS data exists in both forms.
  3. Duplicate entries in the binary form are invalid; blocks with duplicate field values should be rejected. (Note that it is common for Protobuf decoders to accept repeated field values in the binary data, and interpret them as updates to fields that have already been set; DAG-PB is stricter than this.)
  4. Fields and wire types other than those that appear in the Protobuf schema above are invalid and blocks containing these should be rejected. (Note that it is common for Protobuf decoders to skip data in each message type that does not match the fields in the schema.)

Logical Format

When we handle DAG-PB content at the Data Model level, we treat these objects as maps.

This layout can be expressed with IPLD Schemas as:

type PBNode struct {
  Links [PBLink]
  Data optional Bytes
}

type PBLink struct {
  Hash Link
  Name optional String
  Tsize optional Int
}

Constraints

  • The first node in a block of DAG-PB data will match the PBNode type.
  • Data may be omitted or a byte array with a length of zero or more.
  • Links:
    • must be present, even if empty; the binary form makes no distinction between an empty array and an omitted value, in the Data Model we always instantiate an array.
    • elements must be sorted in ascending order by their Name values, which are compared by bytes rather than as strings.
      • Names must be unique or be omitted.
  • Hash:
    • even though Hash is optional in the Protobuf encoding, it should not be treated as optional when creating new blocks or decoding existing ones, an omitted Hash should be interpreted as a bad block
    • the bytes in the encoding format is interpreted as the bytes of a CID, if the bytes cannot be converted to a CID then it should be treated as a bad block.
    • the data is encoded in the binary form as a byte array, it is therefore possible for a decoder to read a correct binary form but fail to convert a Hash to a CID and therefore treat it as a bad block.
  • When creating data, you can create maps using the standard Data Model concepts, and as long as they have exactly these fields. If additional fields are present, the DAG-PB codec will error, because there is no way to encode them.

Both the most recent JavaScript and Go implementations strictly expose this logical format via the Data Model and do not support alternative means of resolving paths via named links as the legacy implementations do (see below).

Alternative (Legacy) Pathing

While the logical format implicitly describes a set of mechanisms for pathing over and through DAG-PB data in strict Data Model form, legacy implementations afford a means of resolving paths by privileging the Name in links.

This alternative pathing is covered here as part of this descriptive spec, but was developed independently of the Data Model and is thus not well standardized. The alternative pathing mechanisms differ between implementations and has been removed from the newer implementations entirely.

The legacy Go and JavaScript implementations both support pathing with link names: /<name1>/<name2>/….

In the legacy Go implementation, this is the only way, which implies that is is impossible to path through nodes that don't name their links. Also neither the Data section nor the Links section/metadata are accessible through paths.

In the legacy JavaScript implementation, there is an additional way to path through the data. It's based purely on the structure of object, i.e. /Links/<index>/Hash/…. This way you have direct access to the Data, Links, and size fields, e.g. /Links/<index>/Hash/Data.

These two ways of pathing can be combined, so you can access e.g. the Data field of a named link via /<name/Data. You can also use both approaches within a single path, e.g. /<name1>/Links/0/Hash/Data or /Links/<index>/Hash/<name>/Data. When using the DAG API in js-ipfs, then the pathing over the structure has precedence, so you won't be able to use named pathing on a named link called Links, you would need to use the index of the link instead.

Both the most recent JavaScript and Go implementations do not expose novel pathing mechanisms but adhere strictly to the IPLD Data Model as described in the above Logical Format schema.

Zero-length blocks

The zero-length DAG-PB block is valid and will be decoded as having null Data and an empty Links array.

With a SHA2-256 multihash, the CID of this block is:

  • CIDv1: bafybeihdwdcefgh4dqkjv67uzcmw7ojee6xedzdetojuzjevtenxquvyku
  • CIDv0: QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n