When configuring QoS classification for FTP, the QoS rules differ depending on whether the FTP data channel is using active or passive FTP. Active versus passive FTP determines whether the FTP client or the FTP server select the port connection for use with the data channel, which has implications for QoS classification.

The application-based shaping doesn’t support passive FTP. Because passive FTP uses random high TCP port numbers to set up its data channel from the FTP server to the FTP client, the FTP data traffic can’t be classified on the TCP port numbers. To classify passive FTP traffic, you can add an application rule where the application is FTP and matches the IP address of the FTP server.

With active FTP, the FTP client logs in and enters the PORT command, informing the server which port it must use to connect to the client for the FTP data channel. Next, the FTP server initiates the connection toward the client. From a TCP perspective, the server and the client swap roles. The FTP server becomes the client because it sends the SYN packet, and the FTP client becomes the server because it receives the SYN packet.

Although not defined in the RFC, most FTP servers use source port 20 for the active FTP data channel.

For active FTP, configure a QoS rule on the server-side SteelHead to match source port 20. On the client-side SteelHead, configure a QoS rule to match destination port 20.

You can also use application definitions to classify active FTP traffic.

With passive FTP, the FTP client initiates both connections to the server. First, it requests passive mode by entering the PASV command after logging in. Next, it requests a port number for use with the data channel from the FTP server. The server agrees to this mode, selects a random port number, and returns it to the client. Once the client has this information, it initiates a new TCP connection for the data channel to the server-assigned port. Unlike active FTP, there’s no role swapping and the FTP client initiates the SYN packet for the data channel. The FTP client receives a random port number from the FTP server.

The QoS Classification configuration for passive FTP is the same as active FTP. Except that when configuring QoS Classification for passive FTP, port 20 on both the server-side and client-side appliances indicates the port number used by the data channel, as opposed to the literal meaning of source or destination port 20.