Failure
                              |
       +-----+      +-----+   V   +-----+      +-----+
       |  A  |------|  B  |--XXX--|  C  |------|  D  |
       +-----+      +-----+       +-----+      +-----+

           <===========|             |============>
             Forward                    Forward
             Defect                     Defect
             Indication                 Indication