Protocol Layering

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

1. Application
2. Transport
3. Network
4. Link
5. Physical