It is essential that all devices that implement any networking protocol do so in a consistent manner. Otherwise, one device might behave in a manner the other wouldn't expect. Naturally, this is why there are standards that describe the operation of each protocol. The problem with a protocol like TCP is that it performs so many tasks that it is difficult to specify the exact operation of all aspects of the protocol succinctly.

One way that computer scientists explain how a complex protocol works is through a theoretical tool called a finite state machine (FSM). An FSM attempts to describe a protocol or algorithm by considering it like a virtual “machine” that progresses through a series of stages of operation in response to various happenings. You need to understand four essential concepts to comprehend the workings of a finite state machine:

• State: The particular “circumstance” or “status” that describes the protocol software on a machine at a given time.
  
  
• Transition: The act of moving from one state to another.
  
  
• Event: Something that causes a transition to occur between states.
  
  
• Action: Something a device does in response to an event before it transitions to another state.

A FSM describes the protocol by explaining all the different states the protocol can be in, the events that can occur in each state, what actions are taken in response to the events and what transitions happen as a result.The protocol usually starts in a particular beginning state when it is first run. It then follows a sequence of steps to get it into a regular operating state, and moves to other states in response to particular types of input or other circumstances. The state machine is called finite because there are only a limited number of states.