Under what circumstances may coarsegrained timeouts still oc

Under what circumstances may coarse-grained timeouts still occur in TCP even when the fast retransmit mechanism is being used?

Solution

Coarse grain timeout involves a coarse grained timer. It means that there is an arbitrary value ‘x’ for the timer. It is then checked every ‘x’ units of time whether a packet that was sent was received or not.

When fast retransmit mechanism is deployed, it requires three duplicate ACKs before retransmitting a packet.

Now consider the following cases:

In such circumstances, even 3 duplicate ACK packets may not reach back to the sender.

Thus, the retransmit mechanism fails and timeout occurs.

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In practice, fast retransmit may eliminate the coarse grain timeouts by approximately 50% and give around 20% improvement in throughput.