T1                T2
         /                   \
+-------+       Query         +-------+
|       | - - - - - - - - - ->|       |
|   Q1  |=====================|   R1  |
|       |<- - - - - - - - - - |       |
+-------+       Response      +-------+
         \                   /
          T4                T3
 Querier                       Responder