Fast Retransmit
Fast Retransmit retransmits a lost segment immediately upon the third duplicate ACK, without waiting for a timeout, accelerating recovery.
Purpose
It recovers quickly from mild loss instead of waiting for the RTO to expire, cutting recovery delay.
Mechanism
When the sender receives 3 duplicate ACKs (implying later segments arrived while one went missing), it immediately retransmits the suspected lost segment, sets ssthresh = cwnd/2, and enters fast recovery.
Use cases
- Fast TCP loss recovery
- Avoiding RTO waits
- Congestion control
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