tcp: more accurately check DSACKs to grow RACK reordering window
Previously, a DSACK could expand the RACK reordering window when no
reordering has been seen, and/or when the DSACK was due to an
unnecessary TLP retransmit (rather than a spurious fast recovery due
to reordering). This could result in unnecessarily growing the RACK
reordering window and thus unnecessarily delaying RACK-based fast
recovery episodes.
To avoid these issues, this commit tightens the conditions under which
a DSACK triggers the RACK reordering window to grow, so that a
connection only expands its RACK reordering window if:
(a) reordering has been seen in the connection
(b) a DSACKed range does not match the most recent TLP retransmit
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Priyaranjan Jha <priyarjha@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 98408d5..3f7bd7a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1001,7 +1001,14 @@ static u32 tcp_dsack_seen(struct tcp_sock *tp, u32 start_seq,
return 0;
tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
- tp->rack.dsack_seen = 1;
+ /* We increase the RACK ordering window in rounds where we receive
+ * DSACKs that may have been due to reordering causing RACK to trigger
+ * a spurious fast recovery. Thus RACK ignores DSACKs that happen
+ * without having seen reordering, or that match TLP probes (TLP
+ * is timer-driven, not triggered by RACK).
+ */
+ if (tp->reord_seen && !(state->flag & FLAG_DSACK_TLP))
+ tp->rack.dsack_seen = 1;
state->flag |= FLAG_DSACKING_ACK;
/* A spurious retransmission is delivered */
