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tcp: Rack at times can miscalculate the RTT from what it thinks is a persists probe respone.

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Turns out that if a peer sends in a window update right after rack fires off
a persists probe, we can mis-interpret the window update and calculate
a bogus RTT (very short). We still process the window update and send
the data but we incorrectly generate an RTT. We should be only doing
the RTT stuff if the rwnd is still small and has not changed.

Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D32717
This commit is contained in:
Randall Stewart 2021-10-29 03:17:43 -04:00
parent 68bff4a07e
commit aeda852782
1 changed files with 14 additions and 7 deletions
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21
sys/netinet/tcp_stacks/rack.c
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@ -5363,8 +5363,6 @@ rack_get_persists_timer_val(struct tcpcb *tp, struct tcp_rack *rack)
t = (tp->t_srtt + (tp->t_rttvar << 2));
RACK_TCPT_RANGESET(tt, t * tcp_backoff[tp->t_rxtshift],
rack_persist_min, rack_persist_max, rack->r_ctl.timer_slop);
if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
tp->t_rxtshift++;
rack->r_ctl.rc_hpts_flags |= PACE_TMR_PERSIT;
ret_val = (uint32_t)tt;
return (ret_val);
@ -14448,11 +14446,20 @@ rack_do_segment_nounlock(struct mbuf *m, struct tcphdr *th, struct socket *so,
* at least use timestamps if available to validate).
*/
rack->forced_ack = 0;
us_rtt = us_cts - rack->r_ctl.forced_ack_ts;
if (us_rtt == 0)
us_rtt = 1;
rack_apply_updated_usrtt(rack, us_rtt, us_cts);
tcp_rack_xmit_timer(rack, us_rtt, 0, us_rtt, 3, NULL, 1);
if (tiwin == tp->snd_wnd) {
/*
* Only apply the RTT update if this is
* a response to our window probe. And that
* means the rwnd sent must match the current
* snd_wnd. If it does not, then we got a
* window update ack instead.
*/
us_rtt = us_cts - rack->r_ctl.forced_ack_ts;
if (us_rtt == 0)
us_rtt = 1;
rack_apply_updated_usrtt(rack, us_rtt, us_cts);
tcp_rack_xmit_timer(rack, us_rtt, 0, us_rtt, 3, NULL, 1);
}
}
/*
* This is the one exception case where we set the rack state