SOA parameters for zone transfer (2/2)

Serial number
the "version number" of the zone
used by the secondary for detecting the need for AXFR
Refresh interval
interval of periodic zone transfer
recommended value: 1 to several hours (RFC1912)
Retry interval
interval for retrying AXFR if the first attempt fails
recommended value: some fraction of refresh interval
Expire
if AXFR keeps failing, how long the secondary will treat its copy as valid
recommended value: 2-4 weeks