tcp.7: Document /proc file tcp_frto_response (new in Linux 2.6.22)

Text taken from Documentation/networking/ip-sysctl.txt

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/tcp.7

@@ -357,6 +357,39 @@ where packet loss is typically due to random radio interference
 rather than intermediate router congestion.
 See RFC 4138 for more details.
 .TP
+.IR tcp_frto_response " (integer; default: 0; since Linux 2.6.22)"
+When F-RTO has detected that a TCP retransmission timeout was spurious
+(i.e, the timeout would have been avoided had TCP set a
+longer retransmission timeout),
+TCP has several options concerning what to do next.
+Possible values are:
+.RS
+.IP 0 3
+Rate halving based; a smooth and conservative response,
+results in halved congestion window
+.RI ( cwnd )
+and slow-start threshold
+.RI ( ssthresh )
+after one RTT.
+.IP 1
+Very conservative response; not recommended because even
+though being valid, it interacts poorly with the rest of
+Linux TCP; halves
+.I cwnd
+and
+.I ssthresh
+immediately.
+.IP 2
+Aggressive response; undoes congestion-control measures
+that are now known to be unnecessary
+(ignoring the possibility of a lost retransmission that would require
+TCP to be more cautious);
+.I cwnd
+and
+.I ssthresh
+are restored to the values prior to timeout.
+.RE
+.TP
 .IR tcp_keepalive_intvl " (integer; default: 75; since Linux 2.4)"
 .\" Since 2.3.18
 The number of seconds between TCP keep-alive probes.