A sender-side D-SACK patch

[Noritoshi Demizu]

Below is a patch for sender-side D-SACK.

Currently, DragonFlyBSD-current (head?) treats D-SACK segments as
normal duplicate ACKs.  This behavior reduces TCP performance in
some cases.  With this patch, D-SACK segments are not treated as
duplicate ACKs.  This patch improves TCP performance in some cases.

The following page shows one case where TCP performance is improved.

  http://www.demizu.org/~noritosi/memo/2005/0728/

Thanks.

Regards,
Noritoshi Demizu


Index: netinet/tcp_input.c
===================================================================
RCS file: /home/cvsup/DragonFlyBSD/dcvs/src/sys/netinet/tcp_input.c,v
retrieving revision 1.60
diff -u -r1.60 tcp_input.c
--- netinet/tcp_input.c	10 May 2005 15:48:10 -0000	1.60
+++ netinet/tcp_input.c	28 Jul 2005 08:09:02 -0000
@@ -1895,7 +1895,8 @@
 		if (SEQ_LEQ(th->th_ack, tp->snd_una)) {
 			if (TCP_DO_SACK(tp))
 				tcp_sack_update_scoreboard(tp, &to);
-			if (tlen != 0 || tiwin != tp->snd_wnd) {
+			if (tlen != 0 || tiwin != tp->snd_wnd ||
+			    (to.to_flags & TOF_DSACK)) {
 				tp->t_dupacks = 0;
 				break;
 			}
Index: netinet/tcp_sack.c
===================================================================
RCS file: /home/cvsup/DragonFlyBSD/dcvs/src/sys/netinet/tcp_sack.c,v
retrieving revision 1.2
diff -u -r1.2 tcp_sack.c
--- netinet/tcp_sack.c	9 Mar 2005 06:54:34 -0000	1.2
+++ netinet/tcp_sack.c	28 Jul 2005 08:09:02 -0000
@@ -261,9 +261,10 @@
 	int startblock;
 	int i;
 
-	if (tcp_sack_ndsack_blocks(blocks, numblocks, tp->snd_una) > 0)
+	if (tcp_sack_ndsack_blocks(blocks, numblocks, tp->snd_una) > 0) {
 		startblock = 1;
-	else
+		to->to_flags |= TOF_DSACK;
+	} else
 		startblock = 0;
 
 	for (i = startblock; i < numblocks; i++) {
Index: netinet/tcp_var.h
===================================================================
RCS file: /home/cvsup/DragonFlyBSD/dcvs/src/sys/netinet/tcp_var.h,v
retrieving revision 1.35
diff -u -r1.35 tcp_var.h
--- netinet/tcp_var.h	10 May 2005 15:48:10 -0000	1.35
+++ netinet/tcp_var.h	28 Jul 2005 08:09:02 -0000
@@ -408,6 +408,7 @@
 #define	TOF_SCALE		0x0020
 #define	TOF_SACK_PERMITTED	0x0040
 #define	TOF_SACK		0x0080
+#define	TOF_DSACK		0x0100
 	u_int32_t	to_tsval;
 	u_int32_t	to_tsecr;
 	tcp_cc		to_cc;		/* holds CC or CCnew */