[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: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index cb715ea..d894bbc 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2587,14 +2587,15 @@
*/
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 @@
} 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);
