For future references, search on the github. https://github.com/pmmp/PocketMine-MP Follow this directory path: "src/pocketmine/network/protocol/Info.php" Search for "const TRANSFER_PACKET". Make sure it's always the right version for your PocketMine. (This branch is 1.0.3. Search for hte 1.0.4 branch.)
If you are developing a plugin, please refrain from setting up custom packets while it already exists in the core. Actually, you should not even use dataPacket() if an API method for the action exists in the core. https://github.com/pmmp/PocketMine-MP/pull/355