FTP protocol discussion FTP protocol discussion FTP typically uses two paths for normal operations. This makes it particularly interesting when working or troubleshooting in NAT environments, Firewall environments, and any packet filtering schemes. Note that each packet after the first one has the ACK bit set. This is important to understanding how the Established keyword works in IOS access-lists.
Commands flow on the command connection, and data flows on the data connection.
Commands such as 'dir' or 'get' or 'put' involve transfers of larger amounts of data, and will invoke the data connection. This is all fairly straight forward so far. But let's talk a little bit about when the connections get setup. The FTP client initiates the command connection to the server. Then in just straight FTP, the server initiates the data connection back to the client.