Using Active and Passive Clients
You can use two data connection mechanisms with the FTP protocol:
Active client The client sends a PASV protocol command from the client to the server. The FTP server then sends the port number of its process to the client, and the server listens for the client to make a connection.
Passive client The client sends the port number of its process to the server. The server sends an acknowledgment when the information is received, and the client listens for the server to make a connection.
You can choose an active or passive client in RWFtpClient methods that initiate data transfer:
If you specify a negative number, the connection is made with an active client, and the port number is chosen algorithmically. If you specify 0 (the default), the connection is made with a passive client. If you specify a positive number, the connection is made with an active client.
