The Layer Relationship

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The Layer Relationship

Between each layer is a common boundary layer. For instance, between the Network layer and the Transport layer is a boundary that both must be able to support. It is through these boundary layers that one layer of the networking model communicates and shares valuable and necessary information with the layer above or below it. In fact, each time a layer passes data to the layer below, it adds information to it, and each time a layer receives data it strips off its own information and passes the rest up the protocol stack. Figure 2.11 illustrates how each layer of the networking model adds and then strips away information.

One of the most common and useful analogies used to describe the networking model is to imagine the process a letter goes through to get to its destination. Figure 2.12 shows a sample of the process.

After looking at Figure 2.12, what would happen if any of these steps broke down? The letter would not be received. The same thing happens on computer networks such as the one in Figure 2.11. If any of the steps in the process break down, messages are not received. Error checking is applied to the model to keep the communications process from breaking down, just as the postal service runs in any kind of weather. But sometimes packets still get lost in the shuffle.

Messages sent from one computer to another move in the same manner. Messages from one layer are packaged and placed into the next layer. Each step of the process has little to do with the preceding or following step. The kind of envelope used has nothing to do with whether you wrote the message in English, French, or German, and it certainly doesn’t matter what the message was. In the same way, where you actually address the envelope, California, Florida, or Hawaii, has absolutely nothing to do with what kind of envelope you use. The only common link between the address and the message is the envelope itself.

Lastly, it doesn’t matter which vehicle: boat, plane or train, the postal service uses to deliver the envelope to its destination address, as long as it gets there. Each layer depends upon the other layers, but is only mildly related in terms of functionality to the others. With this introduction to networks and networking, the architecture of TCP/IP can be both more easily understood and appreciated.

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