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