From: Sepherosa Ziehau <sephe@dragonflybsd.org>
Date: Fri, 15 Jun 2012 09:54:59 +0000 (+0800)
Subject: tcp: Add XMITNOW which bypasses the Nagle algorithm temporarily
X-Git-Tag: v3.2.0~755^2~8
X-Git-Url: https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff_plain/c1fabe8562a4226eed3605179909e4cf398eb6df

tcp: Add XMITNOW which bypasses the Nagle algorithm temporarily

This flag acts differently from ACKNOW that no pure ACK will be sent.
It is currently used by the (extended) limited transmit and the SACK
based fast recovery.

This flag is intended to fix the following bug in the SACK based fast
recovery:
The NextSeg() requires that if the unACKed segments could not pass
IsLost(), previously unsent segment should be selected.  In the
application limited period, the size of the previously unsent segment
could be less than the MSS, thus it could not be sent immediately
according to the Nagle algorithm.  In our SACK based fast recovery
implementation, if the tcp_output() sends no segments, the current
recovery transmit process will stop immediately.  This could stop ACK
clock and cause timeout retransmit, which could be avoided, if the
Nagle algorithm is bypassed temporarily for the small unsent segment
selected by NextSeg().

When this flag is used with (extended) limited transmit, certain amount
of spurious early retranmits could be avoided.
---

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index c11c95a3db..81fb6fc08b 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -3281,6 +3281,7 @@ tcp_sack_rexmt(struct tcpcb *tp, boolean_t force)
 		old_snd_max = tp->snd_max;
 		if (nextrexmt == tp->snd_una)
 			tcp_callout_stop(tp, tp->tt_rexmt);
+		tp->t_flags |= TF_XMITNOW;
 		error = tcp_output(tp);
 		if (error != 0) {
 			tp->rexmt_high = old_rexmt_high;
@@ -3341,6 +3342,7 @@ tcp_sack_limitedxmit(struct tcpcb *tp)
 	tp->snd_cwnd = tp->snd_nxt - tp->snd_una +
 	    rounddown(cwnd_left, tp->t_maxseg);
 
+	tp->t_flags |= TF_XMITNOW;
 	tcp_output(tp);
 
 	sent = tp->snd_nxt - next;
@@ -3606,6 +3608,7 @@ fastretransmit:
 		tp->snd_nxt = tp->snd_max;
 		tp->snd_cwnd = ownd +
 		    (tp->t_dupacks - tp->snd_limited) * tp->t_maxseg;
+		tp->t_flags |= TF_XMITNOW;
 		tcp_output(tp);
 
 		if (SEQ_LT(oldsndnxt, oldsndmax)) {
diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c
index 492e33e46e..2b3e6649a9 100644
--- a/sys/netinet/tcp_output.c
+++ b/sys/netinet/tcp_output.c
@@ -306,7 +306,7 @@ again:
 		flags &= ~TH_SYN;
 		off--, len++;
 		if (len > 0 && tp->t_state == TCPS_SYN_SENT) {
-			tp->t_flags &= ~TF_ACKNOW;
+			tp->t_flags &= ~(TF_ACKNOW | TF_XMITNOW);
 			return 0;
 		}
 	}
@@ -450,6 +450,8 @@ again:
 			goto send;
 		if (SEQ_LT(tp->snd_nxt, tp->snd_max))	/* retransmit case */
 			goto send;
+		if (tp->t_flags & TF_XMITNOW)
+			goto send;
 	}
 
 	/*
@@ -554,6 +556,7 @@ again:
 	/*
 	 * No reason to send a segment, just return.
 	 */
+	tp->t_flags &= ~TF_XMITNOW;
 	return (0);
 
 send:
@@ -1065,7 +1068,7 @@ after_th:
 		KASSERT(error != 0, ("no error, but th not set"));
 	}
 	if (error) {
-		tp->t_flags &= ~TF_ACKNOW;
+		tp->t_flags &= ~(TF_ACKNOW | TF_XMITNOW);
 
 		/*
 		 * We know that the packet was lost, so back out the
@@ -1135,7 +1138,7 @@ out:
 		tp->t_flags &= ~TF_RXRESIZED;
 	}
 	tp->last_ack_sent = tp->rcv_nxt;
-	tp->t_flags &= ~TF_ACKNOW;
+	tp->t_flags &= ~(TF_ACKNOW | TF_XMITNOW);
 	if (tcp_delack_enabled)
 		tcp_callout_stop(tp, tp->tt_delack);
 	if (sendalot)
diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index fe1ec9c18c..c1691d8ab9 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -175,7 +175,7 @@ struct tcpcb {
 #define	TF_RXWIN0SENT	0x00080000	/* sent a receiver win 0 in response */
 #define	TF_FASTRECOVERY	0x00100000	/* in Fast Recovery */
 #define	TF_QUEDFIN	0x00200000	/* FIN has been received */
-#define	TF_UNUSED007	0x00400000
+#define	TF_XMITNOW	0x00400000	/* Temporarily override Nagle */
 #define	TF_UNUSED008	0x00800000
 #define	TF_UNUSED009	0x01000000
 #define	TF_FORCE	0x02000000	/* Set if forcing out a byte */