In TCP/IP, fast retransmit and recovery (FRR) is a congestion control algorithm that makes it possible to quickly recover lost data packets. Without FRR, the TCP uses a timer that requires a retransmission timeout if a packet is lost. No new or duplicate packets can be sent during the timeout period. With FRR, if a receiver receives a data segment that is out of order, it immediately sends a duplicate acknowledgement to the sender. If the sender receives three duplicate acknowledgements, it assumes that the data segment indicated by the acknowledgements is lost and immediately retransmits the lost segment. With FRR, time is not lost waiting for a timeout in order for retransmission to begin.
FRR works most efficiently when there are isolated packet losses. It does not work efficiently when there are multiple data packet losses occurring over a short period of time. The fast retransmit/fast recovery algorithm was introduced in 4.3BSD Reno release and is described in RFC 2001 and RFC 2581.
FRR also stands for false rejection rate, a term used in biometric security systems.