tcp: revert F-RTO middle-box workaround

This reverts commit cc663f4d4c97b7297fb45135ab23cfd508b35a77. While fixing
some broken middle-boxes that modifies receive window fields, it does not
address middle-boxes that strip off SACK options. The best solution is
to fully revert this patch and the root F-RTO enhancement.

Fixes: cc663f4d4c97 ("tcp: restrict F-RTO to work-around broken middle-boxes")
Reported-by: Teodor Milkov <tm@del.bg>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 10 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 575d3c1fb6e8..cd8ea972dc65 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1909,7 +1909,6 @@ 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;
 
@@ -1968,17 +1967,15 @@ 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: 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.
+        /* 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.
          */
         tp->frto = net->ipv4.sysctl_tcp_frto &&
-                   (new_recovery || icsk->icsk_retransmits) &&
                    !inet_csk(sk)->icsk_mtup.probe_size;
 }