The sequence number field (SEQ) is part of the TCP header. There are 4 bytes available for the sequence numbers. The sequence numbers of the sent packets for the virtual connection are entered in the sequence number data field. These are incremented and are used for flow control and the orderly sequence of the data packets. Each byte transmitted by the Transmission Control Protocol (TCP) is assigned a sequence number by the sender, which always represents the first byte in the data packet.
The sender and receiver exchange an arbitrary number during the connection setup, which is entered as the sequence number. The first number is called the Initial Sequence Number( ISN). The first data packet following the connection establishment is a number one digit higher than the ISN number: ISN+1. The sequence numbers are managed by the sender as well as the receiver, each station manages its own.