Refresh, Retry, Expire, Minimum TTL

These settings tell the secondary when to come and retrieve 
the DNS data from the primary, for example, every 6 hours
(REFRESH).  If for some reason the secondary is unable to 
pull at that 6 hour interval then (RETRY) every 1 hour
for up to 20 days (EXPIRE) and if it is unable to pull the 
data from the primary after the 20 days, we should delete 
the DNS data as they are two old.  Also, the primary server 
is telling all caching servers to hold the data for 6 hours
(MINIMUM) and after 6 hours, then to come back and get the 
new data.  Here is an example:

@  IN  SOA  ns.bobspizza.com.  hostmaster.bobspizza.com. (
              2001112024  ; Serial Number 
              21600       ; Refresh every 6 hours
              3600        ; Retry every hour
              1728000     ; Expire every 20 days
              21600 )     ; Minimum 6 hours