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>

1 files changed, 7 insertions, 3 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index b49e41864037..751d39060fb8 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3805,10 +3805,14 @@ invalid_ack:
         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);