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PIP-4: Synchronization Protocol

Table of Contents

Overview

Once two DDRP nodes are connected, they can begin synchronizing blob updates with one another. Since DDRP has no global consensus mechanism, a node is considered "in-sync" when a majority of its peers have no more updates to share.

Concept

At a high level, DDRP's synchronization protocol operates as follows:

  1. A node receives an update notification from one or more of its peers.
  2. The node requests the base of the blob's Merkle tree from one or more of its peers.
  3. The node requests the sectors whose hashes in the tree base differ from those stored by the requesting peer.
  4. The node gossips the update notification to its peers.

Note that the process involves an additional step when nodes connect to the network for the first time. This process is covered in a dedicated section.

To keep track of changes to blobs, DDRP uses a timestamp-based versioning system. When a user updates their blob, they sign both the current timestamp as well as the updated blob's Merkle root. This allows nodes to reject stale updates or updates that happen too often.

Update Notifications

Nodes receive notification of a blob update in one of two ways: from its peers via an Update message, or locally following a CLI-initiated update.

The Update Message

Update messages have the following schema. All fields MUST be set:

  1. type: 0x04

  2. Data:

    1. name: string
    2. timestamp: Time
    3. merkle_root: [32]byte
    4. reserved: [32]byte
    5. signature: Signature

The name field represents the Handshake TLD whose blob is being updated. It MUST be set.

The timestamp field represents the timestamp of the blob's update.

The merkle_root field represents the blob's Merkle root post-update.

The reserved field is reserved for future use. It SHOULD be set to all zeroes. The reserved field is used as part of the blob signature generation process.

The signature field represents the signature of the blob's owner over the blob's timestamp and Merkle root.

Nodes receiving an Update message:

  • MUST disconnect from nodes that send an invalid signature.
  • MUST ignore Update messages with a timestamp value that precedes the referenced blob's timestamp.
  • MUST begin the sector synchronization process if the timestamp value proceeds the referenced blob's timestamp.
  • SHOULD ignore the Update message if the timestamp value is within the one hour window following the referenced blob's timestamp.
  • MUST resolve the provided name to a DDRP key on the Handshake blockchain in order to verify the provided signature.

Sector Synchronization Process

We will describe this process from the perspective of two nodes, Alice and Bob, however in practice sector synchronization SHOULD be carried out among a group of peers. We will describe multiple-peer strategies after describing the protocol itself. The protocol works as follows, assuming that Bob sent Alice the Update message:

The TreeBaseReq Message

The first step in the synchronization process is for Alice to request a tree base from Bob by sending a TreeBaseReq message. It is structured as follows. All fields MUST be set:

  1. type: 0x05

  2. Data:

    1. name: string

The name field represents the Handshake TLD being synchronized.

The TreeBaseRes Message

Once Bob receives the DiffReq message, he responds with a DiffRes message. The DiffRes message is structured as follows, and all fields MUST be set:

  1. type: 0x06

  2. Data:

    1. name: string
    2. tree_base: [256][32]byte

The name field represents the Handshake TLD being synchronized.

The tree_base field represents the base of the Merkle tree as described in PIP-3.

Nodes receiving the TreeBaseRes message:

  • MUST verify the tree_base against the merkle_root received in the Update message.

The SectorReq and SectorRes Messages

With the tree base in hand, Alice begins requesting sector data from Bob for each sector whose hash differs from the ones described in the tree base. She does this by sending SectorReq messages with the following structure:

  1. type: 0x07

  2. Data:

    1. name: string
    2. sector_id: uint16

The name field represents the Handshake TLD for whom sectors are being requested.

The sector_id represents the ID of the sector whose data is being requested.

All fields MUST be set.

Upon receipt of Alice's SectorReq, Bob responds with a SectorRes message with the following structure:

  1. type: 0x08

  2. Data:

    1. name: string
    2. sector_id: uint16
    3. sector: [256]byte

The name field represents the Handshake TLD for whom sectors are being requested.

The sector_id field represents the ID of the sector whose data is being returned.

The sector field represents the returned sector's data.

Nodes receiving SectorRes messages:

  • MUST verify the sector field against the validated tree_base received in the TreeBaseRes message.

Nodes in general:

  • MUST NOT send SectorRes messages for names for which they have no data.

Update Gossip

Nodes MUST send Update messages to their peers once they complete the sector synchronization process. Nodes SHOULD filter out peers that don't need the Update message, such as those that started the sending node's synchronization process.

Initial Synchronization Protocol

When a DDRP node joins the network for the first time, it executes the following protocol to bootstrap blob data:

  1. The node traverses the Handshake blockchain from genesis to head, and aggregates all TLDs that contain DDRPKEY TXT records.
  2. For each TLD with a DDRPKEY record, the node sends an UpdateReq message to its peers.
  3. The node's peers respond with an Update message that contains the information the sending node needs to begin the sector synchronization protocol.
  4. The node begins sector synchronization as described above.

See below for how the UpdateReq message is structured.

The UpdateReq Message

  1. type: 0x0b

  2. Data:

    1. name: string

The name field represents the name for which data is being requested.

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