+------+            +--------+          +--------+
|Client|            |Server A|          |Server B|
+------+            +--------+          +--------+
   |                    |                    |
   |=====QUOTATION=====>|                    |
   |<====PROCESSING=====|                    |
   |                    |=====QUOTATION=====>|
   |                    |<====PROCESSING=====|
   |                    |<=======OFFER=======|
   |                    |=====PROCESSING====>|
   |                    |=======ACCEPT======>|
   |                    |<=======ACK=========|
   |<=======OFFER=======|                    |
   |=====PROCESSING====>|                    |
   |=======ACCEPT======>|                    |
   |<=======ACK=========|                    |
   |                    |                    |