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 upstream proxy can be configured to connect to an NTP server somewhere on the Internet, but this is impossible for the downstream proxy, since it cannot connect to a server that is not part of the downstream network. Therefore the upstream proxy sends its time to the downstream proxy, which can then function as (near) NTP server in the downstream domain (less than one second time difference).
NTP is a standard feature of the Fox DataDiode software.