+---------------------------------------+
                | - Verify time request message.        |
                | - Generate time response message.     |
                |   - Included NTPv4 extension fields:  |
                |      o Unique Identifier EF           |
                |      o NTS Authentication and         |
                |        Encrypted Extension Fields EF  |
                |        - NTS Cookie EF                |
                |        - <NTS Cookie EF>              |
                | - Transmit time request packet.       |
                +-----------------+---------------------+
                                  |
                                  |
Server -----------+---------------+-----+----------------------->
                  ^                      \
                 /                        \
  Time request  /                          \   Time response
  (mode 3)     /                            \  (mode 4)
              /                              \
             /                                V
Client -----+---------------------------------+----------------->
            |                                 |
            |                                 |
            |                                 |
+-----------+-----------------------+   +-----+------------------+
|- Generate time request message.   |   |- Verify time response  |
| - Include NTPv4 Extension fields: |   |  message.              |
|    o Unique Identifier EF         |   |- Extract cookie(s).    |
|    o NTS Cookie EF                |   |- Time synchronization  |
|    o <NTS Cookie Placeholder EF>  |   |  processing.           |
|                                   |   +------------------------+
|- Generate AEAD tag of NTP message.|
|- Add NTS Authentication and       |
|  Encrypted Extension Fields EF.   |
|- Transmit time request packet.    |
+-----------------------------------+