From 5ae83e0d871bc7cbe4dcc9a33d37eb689e631efe Mon Sep 17 00:00:00 2001
From: Michael Tuexen <tuexen@FreeBSD.org>
Date: Thu, 22 Sep 2022 12:12:11 +0200
Subject: [PATCH] tcp: send ACKs when requested

When doing Limited Transmit send an ACK when needed by the protocol
processing (like sending ACKs with a DSACK block).

PR:			264257
PR:			263445
PR:			260393
Reviewed by:		rscheff@
MFC after:		3 days
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D36631
---
 sys/netinet/tcp_input.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index e79cf48b9e5..ac2944a94e3 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -2739,14 +2739,14 @@ enter_recovery:
 					    maxseg;
 					/*
 					 * Only call tcp_output when there
-					 * is new data available to be sent.
-					 * Otherwise we would send pure ACKs.
+					 * is new data available to be sent
+					 * or we need to send an ACK.
 					 */
 					SOCKBUF_LOCK(&so->so_snd);
 					avail = sbavail(&so->so_snd) -
 					    (tp->snd_nxt - tp->snd_una);
 					SOCKBUF_UNLOCK(&so->so_snd);
-					if (avail > 0)
+					if (avail > 0 || tp->t_flags & TF_ACKNOW)
 						(void) tcp_output(tp);
 					sent = tp->snd_max - oldsndmax;
 					if (sent > maxseg) {