+--+                                                     +--+
 |S1|===== \              Forward -->              / =====|R1|
 +--+      \\                                     //      +--+
            \\                                   //
       	    +-----+                             +-----+
       	    |  A  |---------------------------->|  B  |
       	    |     |<----------------------------|     |
       	    +-----+                             +-----+
            //        <-- Backward               \\
+-----+    //                                     \\    +-----+
|S_tcp|=== /                                       \ ===|R_tcp|
+-----+                                                 +-----+