tcp: only postpone PROBE_RTT if RTT is < current min_rtt estimate

From: Ryan Sharpelletti <[email protected]>

During loss recovery, retransmitted packets are forced to use TCP timestamps to calculate the RTT samples, which have a millisecond granularity. BBR is designed using a microsecond granularity. As a result, multiple RTT samples could be truncated to the same RTT value during loss recovery. This is problematic, as BBR will not enter PROBE_RTT if the RTT sample is <= the current min_rtt sample, meaning that if there are persistent losses, PROBE_RTT will constantly be pushed off and potentially never re-entered. This patch makes sure that BBR enters PROBE_RTT by checking if RTT sample is < the current min_rtt sample, rather than <=.

The Netflix transport/TCP team discovered this bug in the Linux TCP BBR code during lab tests.

Fixes 0f8782ea1497 ("tcp_bbr: add BBR congestion control")

Signed-off-by Ryan Sharpelletti <[email protected]>

Signed-off-by Neal Cardwell <[email protected]>

Signed-off-by Soheil Hassas Yeganeh <[email protected]>

Signed-off-by Yuchung Cheng <[email protected]>

---
 net/ipv4/tcp_bbr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

net/ipv4/tcp_bbr.c +1 -2

index 6c4d79baff26..6ea3dc2e4219 100644
@@ -945,7 +945,7 @@ 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 ||
+           (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;
--

2.29.2.299.gdc1121823c-goog