+--------+                                            +--------+
| CLIENT |                                            | SERVER |
|        |Makes connection to 1.2.3.4, port 21.       |        |
|        |>------------------------------------------>|        |
|        |This is the control connection.             |        |
|        |                                            |        |
|        |                                            |        |
|        |Starts listening on a random port.[*]       |        |
|        |                                            |        |
|        |                                            |        |
|        |Sends "PORT 5,6,7,8,45,35"                  |        |
|        |>------------------------------------------>|        |
|        |over the control connection.                |        |
|        |                                            |        |
|        |                                            |        |
|        |    Makes connection to 5.6.7.8, port 11555.|        |
|        |<==========================================<|        |
|        |                This is the data connection.|        |
|        |                                            |        |
|        |                                            |        |
|        |Sends retrieve command ("RETR filename")    |        |
|        |>------------------------------------------>|        |
|        |over the control connection.                |        |
|        |                                            |        |
|        |                                            |        |
|        |                       Sends file "filename"|        |
|        |<==========================================<|        |
|        |                   over the data connection.|        |
|        |                                            |        |
+--------+                                            +--------+