http://www.rfc-editor.org/rfc/rfc7314.txt
RFC 7314: Extension Mechanisms for DNS (EDNS) EXPIRE Option, M. Andrews2014-07-01T23:00:00-00:00This document specifies a method for secondary DNS servers to honour the SOA EXPIRE field as if they were always transferring from the primary, even when using other secondaries to perform indirect transfers and refresh queries.