We first consider the simplest case

We first consider the simplest case, in which the underlying channel is completely
reliable. The protocol itself, which we’ll call rdt1.0, is trivial. The finite-state
machine (FSM) definitions for the rdt1.0 sender and receiver are shown in
Figure 3.9. The FSM in Figure 3.9(a) defines the operation of the sender, while the
FSM in Figure 3.9(b) defines the operation of the receiver. It is important to note
that there are separate FSMs for the sender and for the receiver. The sender and
receiver FSMs in Figure 3.9 each have just one state. The arrows in the FSM
description indicate the transition of the protocol from one state to another. (Since
each FSM in Figure 3.9 has just one state, a transition is necessarily from the one
state back to itself; we’ll see more complicated state diagrams shortly.) The event
causing the transition is shown above the horizontal line labeling the transition, and