Some networks are segregated logically or physically into distinct security or trust domains (e.g. “secret” and “unclassified” domains). Historically such systems were physically separate (air-gapped), but there is increasing demand for controlled transfer of data between them.
From a protective monitoring or audit perspective it is often useful to combine logs from those domains into a single monitoring system without compromising the security of any domain. Two common ways of achieving this are data diodes and guard processors.