+------+                              +------+
|Client|                              |Server|
+------+                              +------+
   |=======QUOTATION(Requested CPD)=====>|
   |<============PROCESSING==============|
   |<=========OFFER(Offered CPD)=========|
   |=============PROCESSING=============>|
   |===============DECLINE==============>|
   |<================ACK=================|
   |                                     |