Secondary                            Primary

       |              NOTIFY               |
       | <-------------------------------- |  UDP
       | --------------------------------> |
       |          NOTIFY Response          |
       |                                   |
       |                                   |
       |            SOA Request            |
       | --------------------------------> |  UDP (or part of
       | <-------------------------------- |  a TCP/TLS session)
       |           SOA Response            |
       |                                   |
       |                                   |
       |                                   |
       |            AXFR Request           | ---
       | --------------------------------> |   |
       | <-------------------------------- |   |
       |          AXFR Response 1          |   |
       |             (Zone data)           |   |
       |                                   |   |
       | <-------------------------------- |   | TLS
       |          AXFR Response 2          |   | Session
       |             (Zone data)           |   |
       |                                   |   |
       | <-------------------------------- |   |
       |          AXFR Response 3          |   |
       |             (Zone data)           | ---
       |                                   |