The Transport Layer's Transport Mechanisms

The Transport Layer offers mechanisms for establishing sessions, multiplexing applications in upper layers, and ending virtual circuits. It provides transparent data transfer by hiding network-dependent details from the higher layers. Both TCP and UDP work at the Transport layer with TCP being more reliable than UDP. When working with TCP/IP protocols, application developers have a choice between the TCP and UDP.

Maintaining Data Integrity with Flow Control

The Transport layer maintains data integrity through flow control and reliable data transport options. Flow control prevents the overflow of a host's buffers, which could lead to lost data. Reliable data transport involves two systems using a connection-oriented communications session to ensure that: segments are acknowledged upon receipt; those that weren't acknowledged are then retransmitted; once segments arrive, they are placed in their proper sequence; and the flow of data is managed to avoid overloads, congestion and loss of data.

A connection-oriented communication session involves a sending device, a receiving device, and a three-way "handshake." The "handshake" involves initiating communication, sending messages, and confirming that the connection has been established and approved. Once the formalities have taken place, the sending device transmits its data. Once complete, the call is terminated and the virtual circuit torn down.

Traffic congestion can occur during data transfers such as when data traffic is generated at a faster pace than the network is capable of handling. When this happens, it's like a traffic jam. There's too much traffic trying to get through, causing a dramatic slowdown.

With traditional traffic bottlenecks, cars eventually get through despite the slowdown. However, with network bottlenecks, a machine receiving too much information, too fast, must store the datagrams in a buffer. If this is just a small burst, buffering will solve the problem. If the data continues flooding the machine, the buffer's capacity will be maxed out and new datagrams will be discarded.

Fortunately the transport function offers flood control systems to avoid this problem. Instead of discarding data, the transport can communicate with the sender, indicating that it's not ready for additional data. Once the buffer is clear, the transport communicates once again, this time signaling that it's ready.

What if each segment had to be acknowledged before the next one sent? The data transmission would be unnecessarily slow. Since there's a window of time between transmitting data and processing acknowledgements, the sending machine can use this window of time to transmit additional data up to a predetermined number of bytes. This process is called "windowing."

What if you sent a party invitation to a friend and your friend acknowledged it by saying he's planning on coming to the party. You'd know that the invitation was received as expected, right? What if your friend never said anything about receiving the invitation and you wanted to be sure that it didn't get lost in the mail. Short of asking, you could always resend the invitation after a set amount of time. Reliable data delivery takes a similar approach. If the receiving machine doesn't acknowledge that the data was received, the sending machine resends the data after a preset amount of time. In addition, once the acknowledgement is received, the sending machine then sends the next segment.

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