From f698204bd0bfdc645642e271da117b56b795aee0 Mon Sep 17 00:00:00 2001 From: Neal Cardwell Date: Wed, 16 Nov 2011 08:58:04 +0000 Subject: [PATCH] tcp: allow undo from reordered DSACKs Previously, SACK-enabled connections hung around in TCP_CA_Disorder state while snd_una==high_seq, just waiting to accumulate DSACKs and hopefully undo a cwnd reduction. This could and did lead to the following unfortunate scenario: if some incoming ACKs advance snd_una beyond high_seq then we were setting undo_marker to 0 and moving to TCP_CA_Open, so if (due to reordering in the ACK return path) we shortly thereafter received a DSACK then we were no longer able to undo the cwnd reduction. The change: Simplify the congestion avoidance state machine by removing the behavior where SACK-enabled connections hung around in the TCP_CA_Disorder state just waiting for DSACKs. Instead, when snd_una advances to high_seq or beyond we typically move to TCP_CA_Open immediately and allow an undo in either TCP_CA_Open or TCP_CA_Disorder if we later receive enough DSACKs. Other patches in this series will provide other changes that are necessary to fully fix this problem. Signed-off-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 15 ++------------- 1 file changed, 2 insertions(+), 13 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 751d39060fb8..a4efdd7cf5a1 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2858,7 +2858,7 @@ static void tcp_try_keep_open(struct sock *sk) struct tcp_sock *tp = tcp_sk(sk); int state = TCP_CA_Open; - if (tcp_left_out(tp) || tcp_any_retrans_done(sk) || tp->undo_marker) + if (tcp_left_out(tp) || tcp_any_retrans_done(sk)) state = TCP_CA_Disorder; if (inet_csk(sk)->icsk_ca_state != state) { @@ -3066,17 +3066,6 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, } break; - case TCP_CA_Disorder: - tcp_try_undo_dsack(sk); - if (!tp->undo_marker || - /* For SACK case do not Open to allow to undo - * catching for all duplicate ACKs. */ - tcp_is_reno(tp) || tp->snd_una != tp->high_seq) { - tp->undo_marker = 0; - tcp_set_ca_state(sk, TCP_CA_Open); - } - break; - case TCP_CA_Recovery: if (tcp_is_reno(tp)) tcp_reset_reno_sack(tp); @@ -3117,7 +3106,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, tcp_add_reno_sack(sk); } - if (icsk->icsk_ca_state == TCP_CA_Disorder) + if (icsk->icsk_ca_state <= TCP_CA_Disorder) tcp_try_undo_dsack(sk); if (!tcp_time_to_recover(sk)) {