Internet Direct (Indy) Version 9.0.2 Beta
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property Serial: cardinal;
Serial is a zone maintenance parameter used to control periodic polling of the secondary name servers for the zone. Serial is used when a secondary name server is attempt to check zone data after Refresh has expired. If this check cannot be completed, new checks are started every Retry seconds.
The check is a simple query to the primary for the TSOARecord Resource Record for the zone. If the value in Serial in the secondary zone data is equal to the Serial returned by Primary, then no changes have occurred, and the Refresh interval wait is restarted. If the secondary finds it impossible to perform a serial check for the Expire interval, it must assume that its copy of the zone is obsolete an discard it.