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tcp: avoid min RTT bloat by skipping RTT from delayed-ACK in BBR

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A persistent connection may send tiny amount of data (e.g. health-check)
for a long period of time. BBR's windowed min RTT filter may only see
RTT samples from delayed ACKs causing BBR to grossly over-estimate
the path delay depending how much the ACK was delayed at the receiver.

This patch skips RTT samples that are likely coming from delayed ACKs. Note
that it is possible the sender never obtains a valid measure to set the
min RTT. In this case BBR will continue to set cwnd to initial window
which seems fine because the connection is thin stream.

Signed-off-by: Yuchung Cheng <[email protected]>
Acked-by: Neal Cardwell <[email protected]>
Acked-by: Soheil Hassas Yeganeh <[email protected]>
Acked-by: Priyaranjan Jha <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
This commit is contained in:
Yuchung Cheng 2018-01-17 12:11:01 -08:00 • committed by David S. Miller
commit e42866031f
3 changed files with 4 additions and 1 deletions
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3
net/ipv4/tcp_bbr.c
View file

@ -766,7 +766,8 @@ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
filter_expired = after(tcp_jiffies32,
bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
if (rs->rtt_us >= 0 &&
(rs->rtt_us <= bbr->min_rtt_us || filter_expired)) {
(rs->rtt_us <= bbr->min_rtt_us ||
(filter_expired && !rs->is_ack_delayed))) {
bbr->min_rtt_us = rs->rtt_us;
bbr->min_rtt_stamp = tcp_jiffies32;
}

1
net/ipv4/tcp_input.c
View file

@ -3633,6 +3633,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
delivered = tp->delivered - delivered; /* freshly ACKed or SACKed */
lost = tp->lost - lost; /* freshly marked lost */
rs.is_ack_delayed = !!(flag & FLAG_ACK_MAYBE_DELAYED);
tcp_rate_gen(sk, delivered, lost, is_sack_reneg, sack_state.rate);
tcp_cong_control(sk, ack, delivered, flag, sack_state.rate);
tcp_xmit_recovery(sk, rexmit);