netdev - [PATCH v2] [TCP]: FRTO undo response falls back to ratehalving one if ECEd

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Message-ID: <Pine.LNX.4.64.0703021430570.3770@kivilampi-30.cs.helsinki.fi>
Date:	Fri, 2 Mar 2007 14:34:36 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	David Miller <davem@...emloft.net>
cc:	netdev@...r.kernel.org
Subject: [PATCH v2] [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@...sinki.fi>
---

Of course I forgot to fix also the high_seq thing I had in mind last 
evening, so here is this again now with it too.

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index dc221a3..6b268dc 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2587,14 +2587,15 @@ static void tcp_conservative_spur_to_res
  */
 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)
@@ -2680,7 +2681,7 @@ static int tcp_process_frto(struct sock 
 		return 1;
 	} else /* frto_counter == 2 */ {
 		switch (sysctl_tcp_frto_response) {
-		case 2: tcp_undo_spur_to_response(sk); break;
+		case 2: tcp_undo_spur_to_response(sk, flag); break;
 		case 1: tcp_conservative_spur_to_response(tp); break;
 		default: tcp_ratehalving_spur_to_response(sk); break;
 		}
-- 
1.4.2