• Open Systems Principles

• Entity is a group of functions for a particular task. Accept input, and produce output.

• Layer is a group of functions designed to provide a set of services related to the mission of that layer

• Entities provide a set of services to the layer above. In doing this they use the services of the underlying layers.

• Each layer isolates the layer above from the details of the underlying layers, and thus a lower layers characteristics can change without affecting the rest of the system.

• Primitives control the layer’s services and data flow. Primitives fall into 4 groups: Request, Indicate, Response, and Confirm


• We can do a simple comparison of networking models