tcp: defer SACK compression after DupThresh

Jean-Louis reported a TCP regression and bisected to recent SACK
compression.

After a loss episode (receiver not able to keep up and dropping
packets because its backlog is full), linux TCP stack is sending
a single SACK (DUPACK).

Sender waits a full RTO timer before recovering losses.

While RFC 6675 says in section 5, "Algorithm Details",

   (2) If DupAcks < DupThresh but IsLost (HighACK + 1) returns true --
       indicating at least three segments have arrived above the current
       cumulative acknowledgment point, which is taken to indicate loss
       -- go to step (4).
...
   (4) Invoke fast retransmit and enter loss recovery as follows:

there are old TCP stacks not implementing this strategy, and
still counting the dupacks before starting fast retransmit.

While these stacks probably perform poorly when receivers implement
LRO/GRO, we should be a little more gentle to them.

This patch makes sure we do not enable SACK compression unless
3 dupacks have been sent since last rcv_nxt update.

Ideally we should even rearm the timer to send one or two
more DUPACK if no more packets are coming, but that will
be work aiming for linux-4.21.

Many thanks to Jean-Louis for bisecting the issue, providing
packet captures and testing this patch.

Fixes: 5d9f4262b7ea ("tcp: add SACK compression")
Reported-by: Jean-Louis Dupond <jean-louis@dupond.be>
Tested-by: Jean-Louis Dupond <jean-louis@dupond.be>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 2 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e695584bb33f..1e37c1388189 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4268,7 +4268,7 @@ static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
          * If the sack array is full, forget about the last one.
          */
         if (this_sack >= TCP_NUM_SACKS) {
-                if (tp->compressed_ack)
+                if (tp->compressed_ack > TCP_FASTRETRANS_THRESH)
                         tcp_send_ack(sk);
                 this_sack--;
                 tp->rx_opt.num_sacks--;
@@ -5189,7 +5189,17 @@ send_now:
         if (!tcp_is_sack(tp) ||
             tp->compressed_ack >= sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr)
                 goto send_now;
-        tp->compressed_ack++;
+
+        if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
+                tp->compressed_ack_rcv_nxt = tp->rcv_nxt;
+                if (tp->compressed_ack > TCP_FASTRETRANS_THRESH)
+                        NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
+                                      tp->compressed_ack - TCP_FASTRETRANS_THRESH);
+                tp->compressed_ack = 0;
+        }
+
+        if (++tp->compressed_ack <= TCP_FASTRETRANS_THRESH)
+                goto send_now;
 
         if (hrtimer_is_queued(&tp->compressed_ack_timer))
                 return;