Suppose a firewall is configured to allow outbound TCP connections but inbound connections only to specified ports. The FTP protocol now presents a problem: when an inside client contacts an outside server, the outbound TCP control connection can be opened normally, but the TCP data connection traditionally is inbound.

(a) Look up the FTP protocol in, for example, Request for Comments 959.

Find out how the PORT command works. Discuss how the client might be written so as to limit the number of ports to which the firewall must grant inbound access. Can the number of such ports be limited to one?

(b) Find out how the FTP PASV command can be used to solve this firewall problem.