Transport Layer--Data Encapsulation Begins

When the data arrives at the transport layer, the protocols at the layer start the process of data encapsulation. The end result depends on whether TCP or UDP has handled the information.

TCP Segmentation

TCP is often called a "connection-oriented" protocol because it ensures the successful delivery of data to the receiving host. Figure 2-1 shows how the TCP protocol receives the stream from the rlogin command. TCP divides the data received from the application layer into segments and attaches a header to each segment.

Segment headers contain sender and recipient ports, segment ordering information, and a data field known as a checksum. The TCP protocols on both hosts use the checksum data to determine whether data has transferred without error.

Establishing a TCP Connection

In addition, TCP uses segments to determine whether the receiving host is ready to receive the data. When the sending TCP wants to establish connections, it sends a segment called a SYN to the peer TCP protocol running on the receiving host. The receiving TCP returns a segment called an ACK to acknowledge the successful receipt of the segment. The sending TCP sends another ACK segment, then proceeds to send the data. This exchange of control information is referred to as a three-way handshake.

UDP Packets

UDP is a "connectionless" protocol. Unlike TCP, it does not check to make sure that data arrived at the receiving host. Instead, UDP takes the message received from the application layer and formats it into UDP packets. UDP attaches a header to each packet, which contains the sending and receiving host ports, a field with the length of the packet, and a checksum.

The sending UDP process attempts to send the packet to its peer UDP process on the receiving host. The application layer determines whether the receiving UDP process acknowledges that the packet was received. UDP requires no notification of receipt. UDP does not use the three-way handshake.