tcp: undo on DSACK during recovery

If the receiver supports DSACK, sender can detect false recoveries and
revert cwnd reductions triggered by either severe network reordering or
concurrent reordering and loss event.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c35b227519827cf583cf840eb85b9c402d1dce2f..907311c9a0121226209a93f381bdd0d6774e5c54 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2315,7 +2315,7 @@ static bool tcp_try_undo_recovery(struct sock *sk)
 }
 
 /* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
-static void tcp_try_undo_dsack(struct sock *sk)
+static bool tcp_try_undo_dsack(struct sock *sk)
 {
        struct tcp_sock *tp = tcp_sk(sk);
 
@@ -2323,7 +2323,9 @@ static void tcp_try_undo_dsack(struct sock *sk)
                DBGUNDO(sk, "D-SACK");
                tcp_undo_cwnd_reduction(sk, false);
                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
+               return true;
        }
+       return false;
 }
 
 /* We can clear retrans_stamp when there are no retransmissions in the
@@ -2751,6 +2753,10 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
                        do_lost = tcp_is_reno(tp) ||
                                  tcp_fackets_out(tp) > tp->reordering;
                }
+               if (tcp_try_undo_dsack(sk)) {
+                       tcp_try_keep_open(sk);
+                       return;
+               }
                break;
        case TCP_CA_Loss:
                tcp_process_loss(sk, flag, is_dupack);