The Microsoft DNS Server in Windows 2000 supports a new kind of zone transfer. Incremental zone transfer, or IXFR for short, is specified in RFC 1995, and it does exactly what you’d expect based on its name. A traditional zone transfer always transfers the entire contents of a zone, even if only one record has changed. Incremental zone transfers allow a name server to send a list of just the records that have changed since the last zone transfer (whether it was a full or incremental one).
This new feature is critical for zones that change frequently. Imagine the scenario with dynamic update: every dynamic update is a change to the zone that requires a zone transfer. Doing a full zone transfer with every small change wastes bandwidth and CPU time. The situation is compounded when the zone being updated and transferred is large.
For IXFR to function, the master servers need to keep track of the differences between successive versions of the zone. A secondary requests an incremental zone transfer and presents its current serial number. The master server calculates and sends the changes needed on the secondary to make its version of the zone current. If the master server can’t calculate the changes for whatever reason—perhaps the secondary has an old version of the zone and the primary hasn’t kept a record of changes that far back—the primary is allowed to say “Sorry, but you’ve got to accept a full zone transfer.”
A Microsoft DNS Server acting as a secondary ...