Explain why it is reasonable to assume that receiving 3 duplicate ACKs in TCP is an indication that the network is not currently congested.
Try an answer before revealing the guidance below.
Key Concepts
- Duplicate ACK
- Later segments delivered
- Isolated loss
- Timeout as severe congestion
Answer Approach
- Ask what event makes the receiver send a duplicate ACK.
- Infer what three duplicates say about the segments sent after the lost one.
- Compare with a timeout, where nothing is getting through.
Full Answer
Answer status: Verified against study notes. Revision notes, not an official marking scheme.
The receiver sends a duplicate ACK each time a segment arrives after a gap. Three duplicate ACKs mean at least three later segments were delivered, so the network path is still carrying traffic; the missing segment was probably an isolated loss (or reordering) rather than the result of heavy congestion. That is why TCP fast-retransmits and only halves cwnd, instead of resetting to slow start as it does after a timeout, when no ACKs are arriving at all.
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