The TCP header is of variable length, consisting of the fields shown in Figure 10-2. When TCP options are not present, the TCP header is 20 bytes long.
Figure 10-2. The structure of the TCP header
The fields in the TCP header are defined as follows:
Source Port. A 2-byte field that indicates the source Application Layer protocol sending the TCP segment. The combination of the source IP address in the IP header and the source port in the TCP header indicates a source socket—a unique, globally significant address from which the segment was sent.
Destination Port. A 2-byte field that indicates the destination Application Layer protocol. ...