1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8
+----------------+----------------+----------------+----------------+
|    action      |    priv_lvl    |  authen_type   | authen_service |
+----------------+----------------+----------------+----------------+
|    user_len    |    port_len    |  rem_addr_len  |    data_len    |
+----------------+----------------+----------------+----------------+
|    user ...
+----------------+----------------+----------------+----------------+
|    port ...
+----------------+----------------+----------------+----------------+
|    rem_addr ...
+----------------+----------------+----------------+----------------+
|    data...
+----------------+----------------+----------------+----------------+