commit ffe7ba4bc229b8622d481aaaa539d8afe630e7f3
Author: Kevin Bowling <kevin.bowling@kev009.com>
Date:   Tue May 19 18:12:48 2026 -0700

    tcp: set initial RTO to 1s per RFC 6298
    
    Lower TCPTV_SRTTDFLT from 3s to 1s and adjust the t_rttvar seed in
    tcp_tcpcb_template() so the pre-measurement TCP_REXMTVAL() equals
    TCPTV_SRTTDFLT (was 2*TCPTV_SRTTDFLT).  Active opens and SYN|ACK
    retransmits now start at 1 s and back off 1, 2, 4, ... per RFC 6298
    sections 2.1 and 5.5.  Post-first-sample RTO is unchanged.
    
    Pin tcp_msl_remote_threshold to its previous numeric default (3*PR_SLOWHZ)
    so the SRTTDFLT change does not move an unrelated TIME_WAIT-shortening
    tunable.
    
    This is conceptually https://reviews.freebsd.org/D18941 which we ran at a
    large CDN, modified for NetBSD's stack.

diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index 38dc53a1d1b0..8b8ed7475b36 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -209,7 +209,7 @@ int	tcp_msl_enable = 1;		/* enable TIME_WAIT truncation	*/
 int	tcp_msl_loop   = PR_SLOWHZ;	/* MSL for loopback		*/
 int	tcp_msl_local  = 5 * PR_SLOWHZ;	/* MSL for 'local'		*/
 int	tcp_msl_remote = TCPTV_MSL;	/* MSL otherwise		*/
-int	tcp_msl_remote_threshold = TCPTV_SRTTDFLT;	/* RTT threshold */
+int	tcp_msl_remote_threshold = 3*PR_SLOWHZ;		/* RTT threshold */
 int	tcp_rttlocal = 0;		/* Use RTT to decide who's 'local' */
 
 int	tcp4_vtw_enable = 0;		/* 1 to enable */
@@ -917,10 +917,10 @@ tcp_tcpcb_template(void)
 
 	/*
 	 * Init srtt to TCPTV_SRTTBASE (0), so we can tell that we have no
-	 * rtt estimate.  Set rttvar so that srtt + 2 * rttvar gives
-	 * reasonable initial retransmit time.
+	 * rtt estimate.  Seed rttvar so the initial RTO computed by
+	 * TCP_REXMTVAL() equals TCPTV_SRTTDFLT (RFC 6298 section 2.1).
 	 */
-	tp->t_rttvar = tcp_rttdflt * PR_SLOWHZ << (TCP_RTTVAR_SHIFT + 2 - 1);
+	tp->t_rttvar = tcp_rttdflt * PR_SLOWHZ << TCP_RTTVAR_SHIFT;
 	TCPT_RANGESET(tp->t_rxtcur, TCP_REXMTVAL(tp),
 	    TCPTV_MIN, TCPTV_REXMTMAX);
 
diff --git a/sys/netinet/tcp_timer.h b/sys/netinet/tcp_timer.h
index ebc0bb0dd765..8d33090cf8c2 100644
--- a/sys/netinet/tcp_timer.h
+++ b/sys/netinet/tcp_timer.h
@@ -119,7 +119,7 @@
 #define	TCPTV_MSL	( 30*PR_SLOWHZ)		/* max seg lifetime (hah!) */
 #define	TCPTV_SRTTBASE	0			/* base roundtrip time;
 						   if 0, no idea yet */
-#define	TCPTV_SRTTDFLT	(  3*PR_SLOWHZ)		/* assumed RTT if no info */
+#define	TCPTV_SRTTDFLT	(  1*PR_SLOWHZ)		/* initial RTO; RFC 6298 (2.1) */
 
 #define	TCPTV_PERSMIN	(  5*PR_SLOWHZ)		/* retransmit persistance */
 #define	TCPTV_PERSMAX	( 60*PR_SLOWHZ)		/* maximum persist interval */