More MTU discovery: avoid over-retransmission if route changes in the

middle of a fully-open window.  Also, keep track of how many retransmits
we do as a result of MTU discovery.  This may actually do more work than
necessary, but it's an unusual condition...

Suggested by: Janey Hoe <janey@lcs.mit.edu>

3 files changed, 22 insertions, 21 deletions

diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index d166bba..0fe5d0f 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tcp_subr.c	8.2 (Berkeley) 5/24/95
- *	$Id: tcp_subr.c,v 1.16 1995/09/22 17:43:37 wollman Exp $
+ *	$Id: tcp_subr.c,v 1.17 1995/10/03 16:54:15 wollman Exp $
  */
 
 #include <sys/param.h>
@@ -503,6 +503,8 @@ tcp_mtudisc(inp, errno)
 		offered = taop->tao_mssopt;
 		mss = rt->rt_rmx.rmx_mtu - sizeof(struct tcpiphdr);
 		mss = min(mss, offered);
+		if (tp->t_maxopd <= mss)
+			return;
 		tp->t_maxopd = mss;
 
 		if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
@@ -523,15 +525,12 @@ tcp_mtudisc(inp, errno)
 
 		tp->t_maxseg = mss;
 
-		/*
-		 * Nudge TCP output.  Unfortunately, we have no way to know
-		 * which packet that we sent is the failing one, but in the
-		 * vast majority of cases we expect that it will be at the
-		 * beginning of the window, so this should do the right
-		 * thing (I hope).
-		 */
-		tp->snd_nxt = tp->snd_una;
-		tcp_output(tp);
+		if (SEQ_GT(tp->snd_una, tp->t_lastmturesend)) {
+			tcpstat.tcps_mturesent++;
+			tp->t_rtt = 0;
+			tp->snd_nxt = tp->t_lastmturesend = tp->snd_una;
+			tcp_output(tp);
+		}
 	}
 }
 #endif /* MTUDISC */
diff --git a/sys/netinet/tcp_timewait.c b/sys/netinet/tcp_timewait.c
index d166bba..0fe5d0f 100644
--- a/sys/netinet/tcp_timewait.c
+++ b/sys/netinet/tcp_timewait.c
@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tcp_subr.c	8.2 (Berkeley) 5/24/95
- *	$Id: tcp_subr.c,v 1.16 1995/09/22 17:43:37 wollman Exp $
+ *	$Id: tcp_subr.c,v 1.17 1995/10/03 16:54:15 wollman Exp $
  */
 
 #include <sys/param.h>
@@ -503,6 +503,8 @@ tcp_mtudisc(inp, errno)
 		offered = taop->tao_mssopt;
 		mss = rt->rt_rmx.rmx_mtu - sizeof(struct tcpiphdr);
 		mss = min(mss, offered);
+		if (tp->t_maxopd <= mss)
+			return;
 		tp->t_maxopd = mss;
 
 		if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
@@ -523,15 +525,12 @@ tcp_mtudisc(inp, errno)
 
 		tp->t_maxseg = mss;
 
-		/*
-		 * Nudge TCP output.  Unfortunately, we have no way to know
-		 * which packet that we sent is the failing one, but in the
-		 * vast majority of cases we expect that it will be at the
-		 * beginning of the window, so this should do the right
-		 * thing (I hope).
-		 */
-		tp->snd_nxt = tp->snd_una;
-		tcp_output(tp);
+		if (SEQ_GT(tp->snd_una, tp->t_lastmturesend)) {
+			tcpstat.tcps_mturesent++;
+			tp->t_rtt = 0;
+			tp->snd_nxt = tp->t_lastmturesend = tp->snd_una;
+			tcp_output(tp);
+		}
 	}
 }
 #endif /* MTUDISC */
diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index bfdb425..759b793 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -31,7 +31,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)tcp_var.h	8.4 (Berkeley) 5/24/95
- * 	$Id: tcp_var.h,v 1.17 1995/09/22 07:40:18 peter Exp $
+ * 	$Id: tcp_var.h,v 1.18 1995/10/04 20:49:03 wollman Exp $
  */
 
 #ifndef _NETINET_TCP_VAR_H_
@@ -141,6 +141,8 @@ struct tcpcb {
 	caddr_t	t_tuba_pcb;		/* next level down pcb for TCP over z */
 /* More RTT stuff */
 	u_long	t_rttupdated;		/* number of times rtt sampled */
+/* For MTU discovery */
+	tcp_seq	t_lastmturesend;	/* seq of resend for mtu discovery */
 };
 
 /*
@@ -287,6 +289,7 @@ struct	tcpstat {
 	u_long	tcps_usedssthresh;	/* times ssthresh initialized from rt*/
 	u_long	tcps_persistdrop;	/* timeout in persist state */
 	u_long	tcps_badsyn;		/* bogus SYN, e.g. premature ACK */
+	u_long	tcps_mturesent;		/* resends due to MTU discovery */
 };
 
 /*