Information older than the SOA expire period should also be purged. An IXFR server may optionally condense multiple difference sequences into a single difference sequence, thus, dropping information on intermediate versions. This may be beneficial if a lot of versions, not all of which are useful, are generated. For example, if multiple ftp servers share a single DNS name and the IP address associated with the name is changed once a minute to balance load between the ftp servers, it is not so important to keep track of all the history of changes. In such a case, server B can not provide incremental data from the unknown version and a full zone transfer is necessary.
Condensation is completely optional.
Clients can't detect from the response whether the server has condensed the reply or not. Given the following three generations of data with the current serial number of 3, example.domain. IN SOA ns.example.domain. rt.example.domain. IN A 10.0.0.1 ftp.example.domain. IN SOA ns.example.domain. rt.example.domain.