When checking to see if a window update should be sent to the remote peer,

don't force a window update if the window would not actually grow due to
window scaling.  Specifically, if the window scaling factor is larger than
2 * MSS, then after the local reader has drained 2 * MSS bytes from the
socket, a window update can end up advertising the same window.  If this
happens, the supposed window update actually ends up being a duplicate ACK.
This can result in an excessive number of duplicate ACKs when using a
higher maximum socket buffer size.

Reviewed by:	bz
MFC after:	1 month

1 files changed, 8 insertions, 0 deletions

diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 55d3c8e7b968..dc2e5ca23f07 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -564,11 +564,19 @@ after_sack_rexmit:
 		long adv = min(recwin, (long)TCP_MAXWIN << tp->rcv_scale) -
 			(tp->rcv_adv - tp->rcv_nxt);
 
+		/* 
+		 * If the new window size ends up being the same as the old
+		 * size when it is scaled, then don't force a window update.
+		 */
+		if ((tp->rcv_adv - tp->rcv_nxt) >> tp->rcv_scale ==
+		    (adv + tp->rcv_adv - tp->rcv_nxt) >> tp->rcv_scale)
+			goto dontupdate;
 		if (adv >= (long) (2 * tp->t_maxseg))
 			goto send;
 		if (2 * adv >= (long) so->so_rcv.sb_hiwat)
 			goto send;
 	}
+dontupdate:
 
 	/*
 	 * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW