TCP is often called a “connection-oriented” protocol because TCP ensures the successful delivery of data to the receiving host. Figure 1–1 shows how the TCP protocol receives the stream from the rlogin command. TCP then divides the data that is received from the application layer into segments and attaches a header to each segment.
Segment headers contain sending and receiving ports, segment ordering information, and a data field that is known as a checksum. The TCP protocols on both hosts use the checksum data to determine if the data transfers without error.