bitcoin-elixir v0.0.1 Bitcoin.Protocol.Messages.GetData
getdata is used in response to inv, to retrieve the content of a specific object, and is usually sent after receiving an inv packet, after filtering known elements. It can be used to retrieve transactions, but only if they are in the memory pool or relay set - arbitrary access to transactions in the chain is not allowed to avoid having clients start to depend on nodes having full transaction indexes (which modern nodes do not).
https://en.bitcoin.it/wiki/Protocol_specification#getdata