Merge branch 'tcp-revert-a-F-RTO-extension-due-to-broken-middle-boxes'

Yuchung Cheng says:

====================
tcp: revert a F-RTO extension due to broken middle-boxes

This patch series reverts a (non-standard) TCP F-RTO extension that aimed
to detect more spurious timeouts. Unfortunately it could result in poor
performance due to broken middle-boxes that modify TCP packets. E.g.
https://www.spinics.net/lists/netdev/msg484154.html
We believe the best and simplest solution is to just revert the change.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 16 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 575d3c1fb6e8..8d480542aa07 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1971,11 +1971,6 @@ void tcp_enter_loss(struct sock *sk)
         /* 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 = net->ipv4.sysctl_tcp_frto &&
                    (new_recovery || icsk->icsk_retransmits) &&
@@ -2631,18 +2626,14 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
             tcp_try_undo_loss(sk, false))
                 return;
 
-        /* The ACK (s)acks some never-retransmitted data meaning not all
-         * the data packets before the timeout were lost. Therefore we
-         * undo the congestion window and state. This is essentially
-         * the operation in F-RTO (RFC5682 section 3.1 step 3.b). Since
-         * a retransmitted skb is permantly marked, we can apply such an
-         * operation even if F-RTO was not used.
-         */
-        if ((flag & FLAG_ORIG_SACK_ACKED) &&
-            tcp_try_undo_loss(sk, tp->undo_marker))
-                return;
-
         if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
+                /* Step 3.b. A timeout is spurious if not all data are
+                 * lost, i.e., never-retransmitted data are (s)acked.
+                 */
+                if ((flag & FLAG_ORIG_SACK_ACKED) &&
+                    tcp_try_undo_loss(sk, true))
+                        return;
+
                 if (after(tp->snd_nxt, tp->high_seq)) {
                         if (flag & FLAG_DATA_SACKED || is_dupack)
                                 tp->frto = 0; /* Step 3.a. loss was real */