Protocol Hierarchies

Networks are organized as a series of layers or levels, to reduce their design complexity. 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.

Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the layer n protocol. Basically, a protocol is an agreement between the communicating parties on how communication is to proceed. The entities comprising the corresponding layers on different machines are called peers. In other words, it is the peers that communicate using the protocol.

In reality, no data directly transferred from Layer n on one machine to layer n on another machine. Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached. Between each pair of adjacent layers there is an interface. The interface defines which primitive operations and services the lower layer offers to he upper one.

A set of layers and protocols is called network architecture. The specification of 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.