The sequence number is used to acknowledge receipt of data. At the beginning of a TCP connection, the client sends a TCP packet with an initial sequence number, but no acknowledgement (there can't be one yet). If there is a server application running at the other end of the connection, the server sends back a TCP packet with its own initial sequence number, and an acknowledgement: the initial sequence number from the client's packet plus one. When the client system receives this packet, it must send back its own acknowledgement: the server's initial sequence number plus one. Thus, it takes three packets to establish a TCP connection (see Part A of Figure 1 which shows the time-line diagram.