file: /proc/sys/net/ipv4/tcp_frto
variable: net.ipv4.tcp_frto
Official reference

Enables Forward RTO-Recovery (F-RTO) defined in RFC5682. F-RTO is an enhanced recovery algorithm for TCP retransmission timeouts. It is particularly beneficial in networks where the RTT fluctuates (e.g., wireless). F-RTO is sender-side only modification. It does not require any support from the peer.

By default it’s enabled with a non-zero value. 0 disables F-RTO.