TCP Header
Source Port (16 bits): Identifies source user process
20 = FTP, 23 = Telnet, 53 = DNS, 80 = HTTP, ...
Destination Port (16 bits)
Sequence Number (32 bits): Sequence number of the first byte in the segment. If SYN is present, this is the initial sequence number (ISN) and the first data byte is ISN+1.
Ack number (32 bits): Next byte expected