PE1 ----- P1 -- P2 -- P3 -- P4 ---- P5 --- P6 --- PE2
    -->    +----+                   +----+   +----+  +----+
  IP Pkt   | IP |                   | IP |   | IP |  | IP |
           +----+                   +----+   +----+  +----+
           |1020|                   |1020|   | 20 |
           +----+                   +----+   +----+
                                    SPRING    LDP