Suppose you want your filter-based firewall to block all incoming Telnet connections, but to allow outbound Telnet connections. One approach would be to block all inbound packets to the designated Telnet port (23).
(a) We might want to block inbound packets to other ports as well, but what inbound TCP connections must be permitted in order not to interfere with outbound Telnet?
(b) Now suppose your firewall is allowed to use the TCP header Flags bits in addition to the port numbers. Explain how you can achieve the desired Telnet effect here while at the same time allowing no inbound TCP connections.