题目:work( ),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 (请作答此空)for these protocols to transparently and securely traverse a firewall.