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!