Sequence Number

After  the bytes have been numbered TCP assigns a sequence number to each segment   that is  being sent. These sequence number for each segment  is the number of the  first  byte  carried  in that  segment.

The  value  in the  sequence  number fields of a segment defines the number of the first  data byte  contained in that  segment when a segment carries a combination of data and control  information. It uses a  sequence number. If a segment  does not  carry  user data. It does  not logically define a sequence number. The field  is there but the value  is not  valid. However some  segments  when carrying  only  control information  needs a sequence  number to allow  an acknowledgment form  the receiver. These segments  are used  for connection  establishment. Termination  or  abortion. Each  of these segment  consumes one sequence number  as through it carried i byte but  there are no actual data. If the randomly  generated  sequence number  is x the first  data  byte  is numbered x +1. The  byte  x is  considered a phony byte  that  is used for a control  segment  to a connection.