tcp: use SACKs and DSACKs that arrive on ACKs below snd_una

The bug: When the ACK field is below snd_una (which can happen when
ACKs are reordered), senders ignored DSACKs (preventing undo) and did
not call tcp_fastretrans_alert, so they did not increment
prr_delivered to reflect newly-SACKed sequence ranges, and did not
call tcp_xmit_retransmit_queue, thus passing up chances to send out
more retransmitted and new packets based on any newly-SACKed packets.

The change: When the ACK field is below snd_una (the "old_ack" goto
label), call tcp_fastretrans_alert to allow undo based on any
newly-arrived DSACKs and try to send out more packets based on
newly-SACKed packets.

Other patches in this series will provide other changes that are
necessary to fully fix this problem.
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3805,10 +3805,14 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	return -1;
 
 old_ack:
+	/* If data was SACKed, tag it and see if we should send more data.
+	 * If data was DSACKed, see if we can undo a cwnd reduction.
+	 */
 	if (TCP_SKB_CB(skb)->sacked) {
-		tcp_sacktag_write_queue(sk, skb, prior_snd_una);
-		if (icsk->icsk_ca_state == TCP_CA_Open)
-			tcp_try_keep_open(sk);
+		flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);
+		newly_acked_sacked = tp->sacked_out - prior_sacked;
+		tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
+				      is_dupack, flag);
 	}
 
 	SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);