From: Sepherosa Ziehau <sephe@dragonflybsd.org>
Date: Wed, 9 May 2012 08:48:15 +0000 (+0800)
Subject: tcp/sack: Don't reduce retransmit threshold as recommended by RFC3517bis
X-Git-Tag: v3.2.0~989
X-Git-Url: https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff_plain/5fd89c20970a436362446e950cb105fb572ee353

tcp/sack: Don't reduce retransmit threshold as recommended by RFC3517bis

- Don't reduce byte threshold in IsLost()
- Don't retransmit if IsLost(tcpcb.snd_una) is true

They cause spurious retransmits.

Add sysctl node net.inet.tcp.rfc3517bis_rxt to enable the RFC3517bis
recommended retransmit threshold reduction.  It is disabled by default.
---

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index 47aac5cacc..837ed1e233 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -201,6 +201,10 @@ int tcp_do_rfc3517bis = 0;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3517bis, CTLFLAG_RW,
     &tcp_do_rfc3517bis, 0, "Enable RFC3517 update");
 
+int tcp_rfc3517bis_rxt = 0;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, rfc3517bis_rxt, CTLFLAG_RW,
+    &tcp_rfc3517bis_rxt, 0, "Enable RFC3517 retransmit update");
+
 SYSCTL_NODE(_net_inet_tcp, OID_AUTO, reass, CTLFLAG_RW, 0,
     "TCP Segment Reassembly Queue");
 
@@ -2029,7 +2033,8 @@ fastretransmit:
 					tp->snd_cwnd += tp->t_maxseg *
 					    (tp->t_dupacks - tp->snd_limited);
 			} else if (tcp_do_rfc3517bis && TCP_DO_SACK(tp)) {
-				if (tcp_sack_islost(&tp->scb, tp->snd_una))
+				if (tcp_rfc3517bis_rxt &&
+				    tcp_sack_islost(&tp->scb, tp->snd_una))
 					goto fastretransmit;
 				if (tcp_do_limitedtransmit) {
 					/* outstanding data */
diff --git a/sys/netinet/tcp_sack.c b/sys/netinet/tcp_sack.c
index c5450414e6..53339b315d 100644
--- a/sys/netinet/tcp_sack.c
+++ b/sys/netinet/tcp_sack.c
@@ -471,7 +471,15 @@ update_lostseq(struct scoreboard *scb, tcp_seq snd_una, u_int maxseg,
 	int bytes_sacked = 0;
 	int rxtthresh_bytes;
 
-	if (tcp_do_rfc3517bis)
+	/*
+	 * XXX
+	 * The RFC3517bis recommends to reduce the byte threshold.
+	 * However, it will cause extra spurious retransmit if
+	 * segments are reordered.  Before certain DupThresh adaptive
+	 * algorithm is implemented, we don't reduce the byte
+	 * threshold (tcp_rfc3517bis_rxt is off by default).
+	 */
+	if (tcp_do_rfc3517bis && tcp_rfc3517bis_rxt)
 		rxtthresh_bytes = (rxtthresh - 1) * maxseg;
 	else
 		rxtthresh_bytes = rxtthresh * maxseg;
diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index c982949116..f15ea56d1d 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -88,6 +88,7 @@ extern int tcp_do_smartsack;
 extern int tcp_do_rescuesack;
 extern int tcp_aggressive_rescuesack;
 extern int tcp_do_rfc3517bis;
+extern int tcp_rfc3517bis_rxt;
 extern int tcp_aggregate_acks;
 extern int tcp_eifel_rtoinc;