Boundary Protection

Monitoring and control of communications at the external boundary of an information system to prevent and detect malicious and other unauthorized communications, through the use of boundary protection devices (e.g., gateways, routers, firewalls, guards, encrypted tunnels). 

See NIST SP 800-53 Rev. 4 under Boundary Protection and CNSSI 4009-2015 (NIST SP 800-53 Rev. 4) for more information.