Fast Retransmit is an enhancement to TCP which reduces the time a sender waits before retransmitting a lost segment.
A TCP sender uses timers to recognize lost segments. If an acknowledgement is not received for a particular segment within a specified time (a function of the estimated Round-trip delay time), the sender will assume the segment was lost in the network, and will retransmit the segment.
The fast retransmit enhancement works as follows: if a TCP sender receives three duplicate acknowledgements with the same acknowledge number (that is, a total of four acknowledgements with the same acknowledgement number), the sender can be reasonably confident that the segment with the next higher sequence number was dropped, and will not arrive out of order. The sender will then retransmit the packet that was presumed dropped before waiting for its timeout.
External links
RFC2001 Documenting Slow Start, Congestion Avoidance, Fast Retransmit & Fast Recovery
RFC2582 The NewReno modification to TCP's Fast Recovery Algorithm
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