tcp: avoid min RTT bloat by skipping RTT from delayed-ACK in BBR (e4286603) · Commits · 戴 / test

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 <ycheng@google.com> Acked-by:

Neal Cardwell <ncardwell@google.com> Acked-by:

Soheil Hassas Yeganeh <soheil@google.com> Acked-by:

Priyaranjan Jha <priyarjha@google.com> Signed-off-by:

David S. Miller <davem@davemloft.net>

include/net/tcp.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -953,6 +953,7 @@ struct rate_sample {
		u32 prior_in_flight; /* in flight before this ACK */
		bool is_app_limited; /* is sample from packet with bubble in pipe? */
		bool is_retrans; /* is sample from retransmission? */
		bool is_ack_delayed; /* is this (likely) a delayed ACK? */
		};

		struct tcp_congestion_ops {

net/ipv4/tcp_bbr.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

net/ipv4/tcp_input.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -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);

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