The Network Time Protocol (NTP) is used to synchronize the clocks on computer systems. Often a so called NTP server is used to provide highly accurate time to clients. Usually, the BLACK proxy can be configured to connect to an NTP server somewhere on the Internet, but this is impossible for the RED proxy, since it cannot connect to a server that is not part of the RED network. Therefore the BLACK proxy sends its time to the RED proxy, which can then function as (near) NTP server in the RED domain (less than one second time difference).
NTP is a standard solution of the Fox DataDiode software.