To design structural network protocols the designers organize protocol and use the network hard ware and software to implement the protocol in layers. Each protocol belong to one of the layers just as each functions in the metro train architecture in figure2.13 belonged to a layer. The services that a layer offers to the above layer is called service model of a layer.
Protocol layering has conceptual and structural advantages. Layering provides a structured way to discuss system components. Modularity makes it easier to update systems components. The first potential drawback of layering is that one layer may duplicate lower layer functionality. For example many protocol stacks provide error recovery on both network basis and point to point procedures. A second potential drawback is that functionality at one layer may need information that is present only in another layer this violates the goal of separation of layers.
The protocols of the various layers are called the protocol stack. The internet protocol stack consists of five layers the physical link network transport and application layers respectively as figure