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authorNeal Cardwell <ncardwell@google.com>2021-07-27 10:42:58 -0400
committerDavid S. Miller <davem@davemloft.net>2021-07-27 20:07:21 +0100
commita657db0350bb8f568897835b6189c84a89f13292 (patch)
tree40699aa991aa192060e472048665088a7029bfd1
parent63f367d9de77b30f58722c1be9e334fb0f5f342d (diff)
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>
-rw-r--r--net/ipv4/tcp_input.c9
-rw-r--r--net/ipv4/tcp_recovery.c3
2 files changed, 10 insertions, 2 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 98408d520c32..3f7bd7ae7d7a 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 */
diff --git a/net/ipv4/tcp_recovery.c b/net/ipv4/tcp_recovery.c
index 6f1b4ac7fe99..fd113f6226ef 100644
--- a/net/ipv4/tcp_recovery.c
+++ b/net/ipv4/tcp_recovery.c
@@ -172,7 +172,8 @@ void tcp_rack_reo_timeout(struct sock *sk)
/* Updates the RACK's reo_wnd based on DSACK and no. of recoveries.
*
- * If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
+ * If a DSACK is received that seems like it may have been due to reordering
+ * triggering fast recovery, increment reo_wnd by min_rtt/4 (upper bounded
* by srtt), since there is possibility that spurious retransmission was
* due to reordering delay longer than reo_wnd.
*