_ _ _ 
          /_______  | | |C|  ACK packet (X)
          \         |_|_|_|
 +---+        layer: 2 3 4 header                            +---+
 |  <|<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Packet X <<<<<<<<<<<<<|<< |L4
 |   |                         +---+                         | ^ |
 |   |                         |  *|>>> Packet W >>>>>>>>>>>>|>^ |L3
 |   |     +---+     +---+     |   |     +---+     +---+     |   |
 |   |     |   |     |   |     |  <|<<<<<|<<<|<(V)<|<<<|     |   |L2
 |   | . . | . |Packet U | . . | . | . . | . | . . | .*| . . |   |L2
 |___|_____|___|_____|___|_____|___|_____|___|_____|___|_____|___|
 source          subnet G      router       subnet H         dest
     __ _ _ _    __ _ _ _    __ _ _        __ _ _ _   later
    |  | | | |  |  | | | |  |  | | |      |  | |C| |  data________\
    |__|_|_|_|  |__|_|_|_|  |__|_|_|      |__|_|_|_|  packet (W)  /
        4 3 2       4 3 2       4 3           4 3 2
                                        _        
                                  /__  |C|  Feedback control
                                  \    |_|  cell/frame (V)
                                        2    
     __ _ _ _    __ _ _ _    __ _ _        __ _ _ _   earlier
    |  | | | |  |  | | | |  |  | | |      |  | | | |  data________\
    |__|_|_|_|  |__|_|_|_|  |__|_|_|      |__|_|_|_|  packet (U)  /
layer:  4 3 2       4 3 2       4 3           4 3 2
header