Secondary                            Primary

       |              NOTIFY               |
       | <-------------------------------- |  UDP
       | --------------------------------> |
       |          NOTIFY Response          |
       |                                   |
       |                                   |
       |            SOA Request            |
       | --------------------------------> |  UDP or TCP
       | <-------------------------------- |  
       |           SOA Response            |
       |                                   |
       |                                   |
       |                                   |
       |            IXFR Request           |
       | --------------------------------> |  UDP or TCP
       | <-------------------------------- |
       |            IXFR Response          |
       |             (Zone data)           |
       |                                   |
       |                                   | ---
       |            IXFR Request           |    |
       | --------------------------------> |    | Retry over
       | <-------------------------------- |    | TCP if 
       |            IXFR Response          |    | required
       |             (Zone data)           | ---