command/response

Both FTP and GridFTP are command/response protocols. What this means is that once a client sends a command to the server, it can only accept responses from the server until it receives a response indicating that the server is finished with that command. For most commands this is not a big deal. For instance, setting the type of the file transfer to binary (called "I" for "image in the protocol"), simply consists of the client sending TYPE I and the server responding with 220 OK. Type set to I. However, the SEND and RETR commands (which actually initiate the movement of data) can run for a long time. Once the command is sent, the client's only options are to wait until it receives the completion reply, or kill the transfer.