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During reliable data transmission each packet is identified by a sequence number in the header. Correctly received packets are acknowledged by the receiver using a control packet that includes the appropriate acknowledgement number. However, in a duplex connection where data packets are sent in both directions, the acknowledgement number can be carried in the next outgoing data packet. The acknowledgement is said to be piggybacked on the data packets.

The application shows piggybacked acknowledgments by a piggy that contains the sequence number and a backpack with the acknowledgement number. Try by yourself if you know the correct numbers for consequent packets!