2.1 The need for layered architecture (protocol hierarchies)

1. To reduce their design complexity, most networks are organised as a stack of layers or levels, each one built upon the one below it. 
2. The number of layers, the name of each layer, the contents of each layer, and the function of each layer differ from network to network. 
3. The purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented. In a sense, each layer is a kind of virtual machine, offering certain services to the layer above it. 
4. A five-layer network is illustrated in Fig. The entities comprising the corresponding layers on different machines are called peers. 
5. The peers may be processes, hardware devices, or even human beings. In other words, it is the peers that communicate by using the protocol.
6. Between each pair of adjacent layers is an interface. The interface defines which primitive operations and services the lower layer makes available to the upper one. 
7. When network designers decide how many layers to include in a network and what each one should do, one of the most important considerations is defining clean interfaces between the layers. 

Figure. Layers, protocols, and interfaces.

8. A set of layers and protocols is called a network architecture. The specification of an architecture must contain enough information to allow an implementer to write the program or build the hardware for each layer so that it will correctly obey the appropriate protocol.