(第4空)The use of network( ),systems that effectively isolate an organization's internal network structure from an( )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.