From cc663f4d4c97b7297fb45135ab23cfd508b35a77 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 7 Apr 2017 11:42:05 -0700 Subject: [PATCH] tcp: restrict F-RTO to work-around broken middle-boxes The recent extension of F-RTO 89fe18e44 ("tcp: extend F-RTO to catch more spurious timeouts") interacts badly with certain broken middle-boxes. These broken boxes modify and falsely raise the receive window on the ACKs. During a timeout induced recovery, F-RTO would send new data packets to probe if the timeout is false or not. Since the receive window is falsely raised, the receiver would silently drop these F-RTO packets. The recovery would take N (exponentially backoff) timeouts to repair N packet losses. A TCP performance killer. Due to this unfortunate situation, this patch removes this extension to revert F-RTO back to the RFC specification. Fixes: 89fe18e44f7e ("tcp: extend F-RTO to catch more spurious timeouts") Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Soheil Hassas Yeganeh Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 20 ++++++++++++-------- 1 file changed, 12 insertions(+), 8 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 2c1f593..659d1ba 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -1935,6 +1935,7 @@ void tcp_enter_loss(struct sock *sk) struct tcp_sock *tp = tcp_sk(sk); struct net *net = sock_net(sk); struct sk_buff *skb; + bool new_recovery = icsk->icsk_ca_state < TCP_CA_Recovery; bool is_reneg; /* is receiver reneging on SACKs? */ bool mark_lost; @@ -1994,15 +1995,18 @@ void tcp_enter_loss(struct sock *sk) tp->high_seq = tp->snd_nxt; tcp_ecn_queue_cwr(tp); - /* F-RTO RFC5682 sec 3.1 step 1 mandates to disable F-RTO - * if a previous recovery is underway, otherwise it may incorrectly - * call a timeout spurious if some previously retransmitted packets - * are s/acked (sec 3.2). We do not apply that retriction since - * retransmitted skbs are permanently tagged with TCPCB_EVER_RETRANS - * so FLAG_ORIG_SACK_ACKED is always correct. But we do disable F-RTO - * on PTMU discovery to avoid sending new data. + /* F-RTO RFC5682 sec 3.1 step 1: retransmit SND.UNA if no previous + * loss recovery is underway except recurring timeout(s) on + * the same SND.UNA (sec 3.2). Disable F-RTO on path MTU probing + * + * In theory F-RTO can be used repeatedly during loss recovery. + * In practice this interacts badly with broken middle-boxes that + * falsely raise the receive window, which results in repeated + * timeouts and stop-and-go behavior. */ - tp->frto = sysctl_tcp_frto && !inet_csk(sk)->icsk_mtup.probe_size; + tp->frto = sysctl_tcp_frto && + (new_recovery || icsk->icsk_retransmits) && + !inet_csk(sk)->icsk_mtup.probe_size; } /* If ACK arrived pointing to a remembered SACK, it means that our -- 2.7.4