+----------------+
         | application: X |
         +----------------+       /  +----------+     +================+
         | authentication-[---->--\--] Linux-   |--<--| PAM config file|
         |       +        [----<--/--]        PAM    |     |================|
         |[conversation()][--+    \  |          |     | X auth .. a.so |
         +----------------+  |    /  +-n--n-----+     | X auth .. b.so |
         |                |  |       __|  |           |           _____/
         |  service user  |  A      |     |           |____,-----' 
         |                |  |      V     A                        
         +----------------+  +------|-----|---------+ -----+------+
                                +---u-----u----+    |      |      |
                                |   auth....   |--[ a ]--[ b ]--[ c ]
                                +--------------+
                                |   acct....   |--[ b ]--[ d ]
                                +--------------+
                                |   password   |--[ b ]--[ c ]
                                +--------------+
                                |   session    |--[ e ]--[ c ]
                                +--------------+