Forward ---->

            +---+          +---+        +---+      +---+
            |S2 |          |R2 |        |S3 |      |R3 |
            +---+          +---+        +---+      +---+
              |              |            |          |
              |              |            |          |
+---+      +-----+       +-----+      +-----+     +-----+      +---+
|S1 |======|  A  |------>|  B  |----->|  C  |---->|  D  |======|R1 |
+---+      |     |<------|     |<-----|     |<----|     |      +---+
           +-----+       +-----+      +-----+     +-----+

                    1st                       2nd
             Bottleneck (A->B)          Bottleneck (C->D)

                          <------ Backward