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>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index b49e418..751d390 100644 (file)
--- 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);