diff mbox

[net] tcp: fix ssthresh and undo for consecutive short FRTO episodes

Message ID 1408047187-983-1-git-send-email-ncardwell@google.com
State Accepted, archived
Delegated to: David Miller
Headers show

Commit Message

Neal Cardwell Aug. 14, 2014, 8:13 p.m. UTC 
Fix TCP FRTO logic so that it always notices when snd_una advances,
indicating that any RTO after that point will be a new and distinct
loss episode.

Previously there was a very specific sequence that could cause FRTO to
fail to notice a new loss episode had started:

(1) RTO timer fires, enter FRTO and retransmit packet 1 in write queue
(2) receiver ACKs packet 1
(3) FRTO sends 2 more packets
(4) RTO timer fires again (should start a new loss episode)

The problem was in step (3) above, where tcp_process_loss() returned
early (in the spot marked "Step 2.b"), so that it never got to the
logic to clear icsk_retransmits. Thus icsk_retransmits stayed
non-zero. Thus in step (4) tcp_enter_loss() would see the non-zero
icsk_retransmits, decide that this RTO is not a new episode, and
decide not to cut ssthresh and remember the current cwnd and ssthresh
for undo.

There were two main consequences to the bug that we have
observed. First, ssthresh was not decreased in step (4). Second, when
there was a series of such FRTO (1-4) sequences that happened to be
followed by an FRTO undo, we would restore the cwnd and ssthresh from
before the entire series started (instead of the cwnd and ssthresh
from before the most recent RTO). This could result in cwnd and
ssthresh being restored to values much bigger than the proper values.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")
---
 net/ipv4/tcp_input.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

Comments

David Miller Aug. 14, 2014, 9:41 p.m. UTC | #1 
From: Neal Cardwell <ncardwell@google.com>
Date: Thu, 14 Aug 2014 16:13:07 -0400

> Fix TCP FRTO logic so that it always notices when snd_una advances,
> indicating that any RTO after that point will be a new and distinct
> loss episode.
> 
> Previously there was a very specific sequence that could cause FRTO to
> fail to notice a new loss episode had started:
> 
> (1) RTO timer fires, enter FRTO and retransmit packet 1 in write queue
> (2) receiver ACKs packet 1
> (3) FRTO sends 2 more packets
> (4) RTO timer fires again (should start a new loss episode)
> 
> The problem was in step (3) above, where tcp_process_loss() returned
> early (in the spot marked "Step 2.b"), so that it never got to the
> logic to clear icsk_retransmits. Thus icsk_retransmits stayed
> non-zero. Thus in step (4) tcp_enter_loss() would see the non-zero
> icsk_retransmits, decide that this RTO is not a new episode, and
> decide not to cut ssthresh and remember the current cwnd and ssthresh
> for undo.
> 
> There were two main consequences to the bug that we have
> observed. First, ssthresh was not decreased in step (4). Second, when
> there was a series of such FRTO (1-4) sequences that happened to be
> followed by an FRTO undo, we would restore the cwnd and ssthresh from
> before the entire series started (instead of the cwnd and ssthresh
> from before the most recent RTO). This could result in cwnd and
> ssthresh being restored to values much bigger than the proper values.
> 
> Signed-off-by: Neal Cardwell <ncardwell@google.com>
> Signed-off-by: Yuchung Cheng <ycheng@google.com>
> Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")

Applied and queued up for -stable, thanks.
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diff mbox

Patch

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 1a8e89f..148d968 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2687,7 +2687,6 @@  static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
  */
 static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
 {
-	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	bool recovered = !before(tp->snd_una, tp->high_seq);
 
@@ -2713,12 +2712,9 @@  static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack)
 
 	if (recovered) {
 		/* F-RTO RFC5682 sec 3.1 step 2.a and 1st part of step 3.a */
-		icsk->icsk_retransmits = 0;
 		tcp_try_undo_recovery(sk);
 		return;
 	}
-	if (flag & FLAG_DATA_ACKED)
-		icsk->icsk_retransmits = 0;
 	if (tcp_is_reno(tp)) {
 		/* A Reno DUPACK means new data in F-RTO step 2.b above are
 		 * delivered. Lower inflight to clock out (re)tranmissions.
@@ -3405,8 +3401,10 @@  static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
 		tcp_rearm_rto(sk);
 
-	if (after(ack, prior_snd_una))
+	if (after(ack, prior_snd_una)) {
 		flag |= FLAG_SND_UNA_ADVANCED;
+		icsk->icsk_retransmits = 0;
+	}
 
 	prior_fackets = tp->fackets_out;