[TCP]: FRTO undo response falls back to ratehalving one if ECEd

Undoing ssthresh is disabled in fastretrans_alert whenever
FLAG_ECE is set by clearing prior_ssthresh. The clearing does
not protect FRTO because FRTO operates before fastretrans_alert.
Moving the clearing of prior_ssthresh earlier seems to be a
suboptimal solution to the FRTO case because then FLAG_ECE will
cause a second ssthresh reduction in try_to_open (the first
occurred when FRTO was entered). So instead, FRTO falls back
immediately to the rate halving response, which switches TCP to
CA_CWR state preventing the latter reduction of ssthresh.

If the first ECE arrived before the ACK after which FRTO is able
to decide RTO as spurious, prior_ssthresh is already cleared.
Thus no undoing for ssthresh occurs. Besides, FLAG_ECE should be
set also in the following ACKs resulting in rate halving response
that sees TCP is already in CA_CWR, which again prevents an extra
ssthresh reduction on that round-trip.

If the first ECE arrived before RTO, ssthresh has already been
adapted and prior_ssthresh remains cleared on entry because TCP
is in CA_CWR (the same applies also to a case where FRTO is
entered more than once and ECE comes in the middle).

High_seq must not be touched after tcp_enter_cwr because CWR
round-trip calculation depends on it.

I believe that after this patch, FRTO should be ECN-safe and
even able to take advantage of synergy benefits.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -2587,14 +2587,15 @@ static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
  */
 static void tcp_ratehalving_spur_to_response(struct sock *sk)
 {
-	struct tcp_sock *tp = tcp_sk(sk);
 	tcp_enter_cwr(sk, 0);
-	tp->high_seq = tp->frto_highmark; 	/* Smoother w/o this? - ij */
 }
 
-static void tcp_undo_spur_to_response(struct sock *sk)
+static void tcp_undo_spur_to_response(struct sock *sk, int flag)
 {
-	tcp_undo_cwr(sk, 1);
+	if (flag&FLAG_ECE)
+		tcp_ratehalving_spur_to_response(sk);
+	else
+		tcp_undo_cwr(sk, 1);
 }
 
 /* F-RTO spurious RTO detection algorithm (RFC4138)
@@ -2681,7 +2682,7 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
 	} else /* frto_counter == 2 */ {
 		switch (sysctl_tcp_frto_response) {
 		case 2:
-			tcp_undo_spur_to_response(sk);
+			tcp_undo_spur_to_response(sk, flag);
 			break;
 		case 1:
 			tcp_conservative_spur_to_response(tp);