_ _ _ 
         /_______  | | |C|  ACK packet (V)
         \         |_|_|_|
+---+        layer: 2 3 4 header                            +---+
|  <|<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Packet V <<<<<<<<<<<<<|<< |L4
|   |                         +---+                         | ^ |
|   | . . .  >>>> Packet U >>>|>>>|>>> Packet U >>>>>>>>>>>>|>^ |L3
|   |     +--^+     +---+     |  v|     +---+     +---+     | ^ |
|   |     |  *|     |   |     |  >|>>>>>|>>>|>>>>>|>>>|>>>>>|>^ |L2
|___|_____|___|_____|___|_____|___|_____|___|_____|___|_____|___|
source          subnet E      router       subnet F         dest
   __ _ _ _|   __ _ _ _|  __ _ _|       __ _ _ _|
  |  | | | |  |  | |C| | |  | |C|      |  | |C|C|    Data________\
  |__|_|_|_|  |__|_|_|_| |__|_|_|      |__|_|_|_|    Packet (U)  /
layer:4 3 2       4 3 2      4 3           4 3 2
header