scope:

In general terms, authoritative name servers for a given zone can use various means to achieve coherency of the zone contents they serve. For example, there are DNS implementations that assemble answers from data stored in relational databases (as opposed to master files), relying on the database's non-DNS means to synchronize the database instances. Some of these non-DNS solutions interoperate in some fashion. However, AXFR, IXFR, and NOTIFY are the only protocoldefined in-band mechanisms to provide coherence of a set of name servers, and they are the only mechanisms specified by the IETF.

This document does not cover incoherent DNS situations. There are applications of the DNS in which servers for a zone are designed to be incoherent. For these configurations, a coherency mechanism as described here would be unsuitable.

A DNS implementation is not required to support AXFR, IXFR, and NOTIFY, but it should have some means for maintaining name server coherency. A general-purpose DNS implementation will likely support AXFR (and in the same vein IXFR and NOTIFY), but turnkey DNS implementations may exist without AXFR.