helloData (1)

The helloData message is the successful response to a hello message. It confirms that the sender has accepted the handshake and provides the receiving peer with its current blockchain status.

Receiving a helloData message signifies that the application-layer handshake is complete and the session is fully established. Peers can then proceed to exchange other protocol messages.

Payload Structure

The helloData payload is composed of two parts:

1. The entire payload of the original hello message. The structure is identical to the one sent by the initiator. See the hello message specification for a detailed breakdown of these fields.
2. Additional fields appended immediately after the hello payload, which describe the sender's current blockchain state.

Core Data Types

• IxiVarInt / IxiVarUInt: See IxiVarInt Encoding.
• IxiBytes: See IxiBytes Encoding.
• string: See Global Serialization Rules.

Appended Fields

Field	Data Type	Min	Max	Description
lastBlockNum	IxiVarUInt	0		The block number of the latest block in the sender's blockchain.
lastBlockChecksum	IxiBytes			The checksum of the latest block.
lastBlockVersion	IxiVarInt			The version of the latest block.
myExternalAddress	string			The recipient's IP address and port, as seen by the sender.

Behavioral Notes

• Handshake Completion: Upon successfully parsing this message, a node should consider the connection fully authenticated and ready for general protocol communication.
• Blockchain Synchronization: The primary purpose of the appended fields is to allow the recipient to quickly compare its own blockchain state with the sender's. If the lastBlockNum or lastBlockChecksum do not match, the node knows it needs to initiate a synchronization process (e.g., by sending getBlockHeaders3).
• NAT Traversal/IP Discovery: The myExternalAddress field serves as a confirmation of the recipient's public-facing network address. This is useful for nodes behind NAT to discover their external IP and determine their network reachability.