How does time-out retransmission differ from fast retransmit?

• A. Time-out retransmission triggers when no ACK is received within a timeout; fast retransmit triggers when duplicate ACKs are received.
• B. Time-out retransmission uses duplicate ACKs; fast retransmit uses a timer.
• C. They are the same.
• D. Time-out retransmission only applies to UDP.

Answer: (A)

The main idea tested is how TCP detects and handles lost data through two different retransmission mechanisms. Time-out retransmission happens when the sender’s retransmission timer expires without seeing an acknowledgment for the segment. That timeout signals a likely loss, so the sender retransmits the unacknowledged data and typically enters a more cautious congestion window adjustment.

Fast retransmit, on the other hand, is triggered earlier by the reception of duplicate acknowledgments. When the receiver keeps acknowledging the last in-order byte and repeats the same acknowledgment, the sender infers that a segment within the window was lost behind what has already been received. After detecting a certain number of duplicate ACKs (commonly three), the sender quickly retransmits the presumed missing segment without waiting for the timer, often followed by a more limited adjustment of the congestion window (fast recovery).

So the correct statement captures that time-out retransmission relies on a timer, while fast retransmit relies on duplicate ACKs to detect loss earlier. The other options misstate who uses the timer, claim they are the same, or wrongly tie time-out retransmission to UDP.