Multiple applications can share the same transport connection in the OSI reference model. Transport functionality is accomplished on a segment-by-segment basis. In other words, different applications can send data segments on a first-come, first-served basis. The segments that arrive first will be taken care of first. These segments can be routed to the same or different destinations. This is referred to as the multiplexing of upper-layer conversations.

One function of the transport layer is to establish a connection-oriented session between similar devices at the application layer. For data transfer to begin, both the sending and receiving applications inform the respective operating systems that a connection will be initiated. One node initiates a connection that must be accepted by the other. Protocol software modules in the two operating systems communicate with each other by sending messages across the network to verify that the transfer is authorized and that both sides are ready.

The connection is established and the transfer of data begins after all synchronization has occurred. During transfer, the two machines continue to communicate with their protocol software to verify that data is received correctly.

Figure shows a typical connection between the sending and receiving systems. The first handshake requests synchronization. The second and third handshakes acknowledge the initial synchronization request, as well as synchronizing connection parameters in the opposite direction. The final handshake segment is an acknowledgment used to inform the destination that both sides agree that a connection has been established. After the connection has been established, data transfer begins.

Congestion can occur during data transfer for two reasons. First, a high-speed computer might be capable of generating traffic faster than a network can transfer it. Second, if many computers simultaneously need to send datagrams to a single destination, that destination can experience congestion, although no single source caused the problem.

When datagrams arrive too quickly for a host or gateway to process, they are temporarily stored in memory. If the traffic continues, the host or gateway eventually exhausts its memory and must discard additional datagrams that arrive.

Instead of allowing data to be lost, the transport function can issue a “not ready” indicator to the sender. Acting like a stop sign, this indicator signals the sender to stop sending data. When the receiver can handle additional data, the receiver sends a “ready” transport indicator. When this indicator is received, the sender can resume the segment transmission.

At the end of data transfer, the sending host sends a signal that indicates the end of the transmission. The receiving host at the end of the data sequence acknowledges the end of transmission and the connection is terminated.

Web Links Layer 3 - Transport Layer http://www.internet-tips.net/Networks/ osi_transport.ht