Each message is assigned a sequence number that increases monotonically on the server side. To save bandwidth, the sequence number is a field in the block header, instead of every message. The sequence number for the first message is used in the header, and the client is expected to derive the sequence numbers for subsequent messages in the block. In the case of a heartbeat which includes only header in the block, the expected next sequence number is used.

public int SequenceNumber { get; }