1. SOA records

   A problem I've found in quite some nameservers is that the various
   timers have been set (far) too low. Especially for top level domain
   nameservers this causes unnecessary traffic over international and
   intercontinental links.

   Unfortunately the examples given in the BIND manual, in RFC's and in
   some expert documents give those very short timer values, and that's
   most likely what people have modeled their SOA records after.

   First of all a short explanation of the timers used in the SOA
   record:

        - Refresh: The SOA record of the primary server is checked
                   every "refresh" time by the secondary servers;
                   if it has changed, a zone transfer is done.

        - Retry: If a secondary server cannot reach the primary
                 server, it tries it again every "retry" time.

        - Expire: If for "expire" time the primary server cannot
                  be reached, all information about the zone is
                  invalidated on the secondary servers (i.e., they
                  are no longer authoritative for that zone).

        - Minimum TTL: The default TTL value for all records in the
                       zone file; a different TTL value may be given
                       explicitly in a record when necessary.
                       (This timer is named "Minimum", and that's
                       what it's function should be according to
                       STD 13, RFC 1035std13, but most (all?)
                       implementations take it as the default value
                       exported with records without an explicit TTL
                       value).

   For top level domain servers I would recommend the following values:

          86400 ; Refresh     24 hours
           7200 ; Retry        2 hours
        2592000 ; Expire      30 days
         345600 ; Minimum TTL  4 days

   For other servers I would suggest:

          28800 ; Refresh     8 hours
           7200 ; Retry       2 hours
         604800 ; Expire      7 days
          86400 ; Minimum TTL 1 day

   but here the frequency of changes, the required speed of propagation,
   the reachability of the primary server etc. play a role in optimizing
   the timer values.