From e317f6f69cb95527799d308a9421b7dc1252989a Mon Sep 17 00:00:00 2001 From: Ilpo Järvinen Date: Fri, 2 Mar 2007 13:34:19 -0800 Subject: [TCP]: FRTO undo response falls back to ratehalving one if ECEd MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index cb715eadf8f5..d894bbcc1d24 100644 --- a/net/ipv4/tcp_input.c +++ b/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); -- cgit v1.2.3-59-g8ed1b