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 <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2c1f59386a7bac8a8d9034ad8c64ba14d877fed2..659d1baefb2bba36d96e412eb7ca5a02996fb6dd 100644 (file)
--- 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