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>

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 */