FSM of modified RDT 2.1 that includes sender timeout and retransmit:
A timer is added whose value is greater than the known round-trip propagation delay. When the timeout event occurs, the most recently transmitted packet is retransmitted.
rdt2.1 receiver is still works even though failure occurs in packet loss.
• When a lost packet causes the timeout, then the receiver will never receive the previous transmission. When the receiver receives the timeout retransmission packet, the receiver assumes that this packet is the original transmission.
• When an ACK is lost, then the receiver will retransmit the packet on a timeout. The retransmission means that the ACK is garbled. Even if the ACK is lost or garbled, the sender reaction is the same.
• Rdt2.1 receiver handles the case of the garbled ACK.
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