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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Teodor Milkov <tm@del.bg>,
        Yuchung Cheng <ycheng@google.com>,
        Neal Cardwell <ncardwell@google.com>,
        "David S. Miller" <davem@davemloft.net>
Subject: [PATCH 4.15 096/122] tcp: revert F-RTO middle-box workaround
Date:   Wed,  7 Mar 2018 11:38:28 -0800
Message-Id: <20180307191743.245437591@linuxfoundation.org>
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4.15-stable review patch.  If anyone has any objections, please let me know.

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

From: Yuchung Cheng <ycheng@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 <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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp_input.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 45f750e85714..50963f92a67d 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1915,7 +1915,6 @@ 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;
 
@@ -1974,17 +1973,15 @@ 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;
 }
 
-- 
2.14.3