Index: sys/netinet/tcp.h
===================================================================
RCS file: /home/ncvs/src/sys/netinet/tcp.h,v
retrieving revision 1.40
diff -p -u -I__FBSDID -I$FreeBSD -r1.40 tcp.h
--- sys/netinet/tcp.h	25 May 2007 21:28:49 -0000	1.40
+++ sys/netinet/tcp.h	18 Jun 2008 05:36:20 -0000
@@ -78,6 +78,8 @@ struct tcphdr {
 
 #define	TCPOPT_EOL		0
 #define	   TCPOLEN_EOL			1
+#define	TCPOPT_PAD		0		/* padding after EOL */
+#define	   TCPOLEN_PAD			1
 #define	TCPOPT_NOP		1
 #define	   TCPOLEN_NOP			1
 #define	TCPOPT_MAXSEG		2
Index: sys/netinet/tcp_output.c
===================================================================
RCS file: /home/ncvs/src/sys/netinet/tcp_output.c,v
retrieving revision 1.141.2.3
diff -p -u -I__FBSDID -I$FreeBSD -r1.141.2.3 tcp_output.c
--- sys/netinet/tcp_output.c	5 Dec 2007 10:37:17 -0000	1.141.2.3
+++ sys/netinet/tcp_output.c	18 Jun 2008 05:36:21 -0000
@@ -1280,12 +1280,16 @@ tcp_addoptions(struct tcpopt *to, u_char
 	for (mask = 1; mask < TOF_MAXOPT; mask <<= 1) {
 		if ((to->to_flags & mask) != mask)
 			continue;
+		if (optlen == TCP_MAXOLEN)
+			break;
 		switch (to->to_flags & mask) {
 		case TOF_MSS:
 			while (optlen % 4) {
 				optlen += TCPOLEN_NOP;
 				*optp++ = TCPOPT_NOP;
 			}
+			if (TCP_MAXOLEN - optlen < TCPOLEN_MAXSEG)
+				continue;
 			optlen += TCPOLEN_MAXSEG;
 			*optp++ = TCPOPT_MAXSEG;
 			*optp++ = TCPOLEN_MAXSEG;
@@ -1298,6 +1302,8 @@ tcp_addoptions(struct tcpopt *to, u_char
 				optlen += TCPOLEN_NOP;
 				*optp++ = TCPOPT_NOP;
 			}
+			if (TCP_MAXOLEN - optlen < TCPOLEN_WINDOW)
+				continue;
 			optlen += TCPOLEN_WINDOW;
 			*optp++ = TCPOPT_WINDOW;
 			*optp++ = TCPOLEN_WINDOW;
@@ -1308,6 +1314,8 @@ tcp_addoptions(struct tcpopt *to, u_char
 				optlen += TCPOLEN_NOP;
 				*optp++ = TCPOPT_NOP;
 			}
+			if (TCP_MAXOLEN - optlen < TCPOLEN_SACK_PERMITTED)
+				continue;
 			optlen += TCPOLEN_SACK_PERMITTED;
 			*optp++ = TCPOPT_SACK_PERMITTED;
 			*optp++ = TCPOLEN_SACK_PERMITTED;
@@ -1317,6 +1325,8 @@ tcp_addoptions(struct tcpopt *to, u_char
 				optlen += TCPOLEN_NOP;
 				*optp++ = TCPOPT_NOP;
 			}
+			if (TCP_MAXOLEN - optlen < TCPOLEN_TIMESTAMP)
+				continue;
 			optlen += TCPOLEN_TIMESTAMP;
 			*optp++ = TCPOPT_TIMESTAMP;
 			*optp++ = TCPOLEN_TIMESTAMP;
@@ -1355,7 +1365,7 @@ tcp_addoptions(struct tcpopt *to, u_char
 				optlen += TCPOLEN_NOP;
 				*optp++ = TCPOPT_NOP;
 			}
-			if (TCP_MAXOLEN - optlen < 2 + TCPOLEN_SACK)
+			if (TCP_MAXOLEN - optlen < TCPOLEN_SACKHDR + TCPOLEN_SACK)
 				continue;
 			optlen += TCPOLEN_SACKHDR;
 			*optp++ = TCPOPT_SACK;
@@ -1386,9 +1396,15 @@ tcp_addoptions(struct tcpopt *to, u_char
 		optlen += TCPOLEN_EOL;
 		*optp++ = TCPOPT_EOL;
 	}
+	/*
+	 * According to RFC 793 (STD0007):
+	 *   "The content of the header beyond the End-of-Option option
+	 *    must be header padding (i.e., zero)."
+	 *   and later: "The padding is composed of zeros."
+	 */
 	while (optlen % 4) {
-		optlen += TCPOLEN_NOP;
-		*optp++ = TCPOPT_NOP;
+		optlen += TCPOLEN_PAD;
+		*optp++ = TCPOPT_PAD;
 	}
 
 	KASSERT(optlen <= TCP_MAXOLEN, ("%s: TCP options too long", __func__));