IXFR
Incremental Zone Transfer: a DNS operation that copies only the records changed since the last transfer, instead of the whole zone.
IXFR (Incremental Zone Transfer) is the efficient sibling of AXFR. Instead of shipping the entire zone every time, a secondary nameserver tells the primary "I have serial 2026051601, send me everything that changed since then." The primary replies with just the added and deleted records. For large zones like .com or busy enterprise zones, IXFR cuts transfer bandwidth by orders of magnitude. If the primary cannot satisfy an incremental request (for example, the serial is too old), it falls back to a full AXFR automatically.