The retransmission timeout should the srtt + 4 * rttvar. The srtt is using 5 bits of fractions and rttvar is using 4 bits of fractions, some specific shifting is required.

Test Plan

Run the following packetdrill scripts:

//
// Copyright (c) 2026 Michael Tuexen
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
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// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
// SUCH DAMAGE.
//
// Ensure that all relevant sysctl variables have their default values.
 0.000 `sysctl -w net.inet.tcp.sack.enable=1`
// Ensure the BBR stack is available.
+0.000 `kldload -n tcp_bbr`

// Get the TCP connection is LAST-ACK state.
+0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0.000 setsockopt(3, IPPROTO_TCP, TCP_FUNCTION_BLK, {function_set_name="bbr",
                                                     pcbcnt=0}, 36) = 0
+0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0.000 bind(3, ..., ...) = 0
+0.000 listen(3, 1) = 0
+0.000 < S   0:0(0)          win 65535 <mss 1460,sackOK,eol,eol>
+0.000 > S.  0:0(0)   ack  1 win 65535 <mss 1460,sackOK,eol,eol>
+0.050 <  .  1:1(0)   ack  1 win 65535
+0.000 accept(3, ..., ...) = 4
+0.000 close(3) = 0
+0.000 setsockopt(4, IPPROTO_TCP, TCP_LOG, [TCP_LOG_STATE_CONTINUAL], 4) = 0
+0.000 send(4, ..., 24, 0) = 24
+0.000 > P.  1:25(24) ack  1 win 65535
+0.050 <  .  1:1(0)   ack 25 win 65535
+0.100 send(4, ..., 24, 0) = 24
+0.000 > P. 25:49(24) ack  1 win 65535
*      > P. 25:49(24) ack  1 win 65535
+0.300 > P. 25:49(24) ack  1 win 65535
+0.050 < F.  1:1(0)   ack 49 win 65535
+0.000 >  . 49:49(0)  ack  2 win 65535
+0.000 close(4) = 0
+0.000 > F. 49:49(0)  ack  2 win 65535
+0.050 <  .  2:2(0)   ack 50 win 65535