work( ),systems that effectively isolate an organization's internal network structure from an (请作答此空) network, such as the INTERNET is becoming increasingly popular. These firewall systems typically act as application-layer( )between networks, usually offering controlled TELNET, FTP, and SMTP access. With the emergence of more sophisticated( )layer protocols designed to facilitate global information discovery , there exists a need to provide a general ( )for these protocols to transparently and securely traverse a firewall. A.exterior B.Internal C.center D.middle