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
+----------------+----------------+----------------+----------------+
|          user_msg len           |            data_len             |
+----------------+----------------+----------------+----------------+
|     flags      |  user_msg ...
+----------------+----------------+----------------+----------------+
|    data ...
+----------------+