Acknowledgement Number - This 32-bit number is the other host’s sequence number + 1 of the last
successfully received byte of data. It is the sequence number of the next expected byte of data. This
field is only valid when the ACK control bit is set. Since sending an ACK costs nothing, (because it
and the Acknowledgement Number field are part of the header) the ACK control bit is always set after
a connection has been established.