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).
Source(s):
CNSSI 4009-2015
Monitoring and control of communications at the external interface to a system to prevent and detect malicious and other unauthorized communications using boundary protection devices.
Source(s):
NIST SP 800-53 Rev. 5