What is it ?

• It defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively
• Provides communication services where the process is used to exchange messages
• When communication is simple, we use only one protocol
• When communication is complex, we divide the task between different layers

First Principle

• If we need bidirectional communication, the layer should be in such a way to be able to perform two opposite tasks

Second Principle

• The two objects in each layer at both sides must be identical