4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Yuchung Cheng <ych...@google.com>


[ Upstream commit d4131f09770d9b7471c9da65e6ecd2477746ac5c ]

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 <t...@del.bg>
Signed-off-by: Yuchung Cheng <ych...@google.com>
Signed-off-by: Neal Cardwell <ncardw...@google.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 net/ipv4/tcp_input.c |   17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1947,7 +1947,6 @@ 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;
 
@@ -2010,17 +2009,15 @@ 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 = sysctl_tcp_frto &&
-                  (new_recovery || icsk->icsk_retransmits) &&
                   !inet_csk(sk)->icsk_mtup.probe_size;
 }
 


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