piece (id 7): a block of data, block bytes at begin within piece. On the wire: [len][7][int32 piece][int32 begin][raw block], len = block.size + 9.
piece
[len][7][int32 piece][int32 begin][raw block]
block.size + 9
The numeric message id, or -1 for KeepAlive (which has no id byte on the wire). Matches bt_peer_connection::message_type.
bt_peer_connection::message_type
The full on-the-wire frame: 4-byte big-endian length prefix + id + payload.