Protocol Heirarchies ~ Protocol Stacks
a sequence of layers defines a network architecture
layer N on one host communicates with layer N on another host - these are called peers
between adjacent pairs of layers is an interface
a service is a set of primitives (operations) that one layer provides to the layer above it
layer N uses the services of layer N-1 while layer N-1 provides this service to layer N
the interaction between peers is distinct from services and the interactions through an interface