diff -Naur sys.dist/conf/options sys/conf/options
--- sys.dist/conf/options	Wed Aug  2 22:19:33 2006
+++ sys/conf/options	Thu Aug 31 16:55:23 2006
@@ -386,6 +386,7 @@
 SLIP_IFF_OPTS		opt_slip.h
 TCPDEBUG
 TCP_SIGNATURE		opt_inet.h
+TCP_TSO			opt_global.h
 TCP_SACK_DEBUG		opt_tcp_sack.h 
 TCP_DROP_SYNFIN		opt_tcp_input.h
 DEV_VLAN		opt_vlan.h
diff -Naur sys.dist/net/if.h sys/net/if.h
--- sys.dist/net/if.h	Mon Jun 19 15:20:44 2006
+++ sys/net/if.h	Thu Aug 31 12:33:40 2006
@@ -189,6 +189,7 @@
 #define	IFCAP_JUMBO_MTU		0x0020	/* 9000 byte MTU supported */
 #define	IFCAP_POLLING		0x0040	/* driver supports polling */
 #define	IFCAP_VLAN_HWCSUM	0x0080	/* can do IFCAP_HWCSUM on VLANs */
+#define	IFCAP_TCPSEG		0x0100	/* can do TSO */
 
 #define IFCAP_HWCSUM		(IFCAP_RXCSUM | IFCAP_TXCSUM)
 
diff -Naur sys.dist/netinet/ip_output.c sys/netinet/ip_output.c
--- sys.dist/netinet/ip_output.c	Thu Jun 29 06:38:36 2006
+++ sys/netinet/ip_output.c	Thu Aug 31 15:16:44 2006
@@ -495,9 +495,11 @@
 
 	/*
 	 * If small enough for interface, or the interface will take
-	 * care of the fragmentation for us, can just send directly.
+	 * care of the fragmentation for us, or the interface is
+	 * doing segmentation, we can just send directly.
 	 */
-	if (ip->ip_len <= ifp->if_mtu || (ifp->if_hwassist & CSUM_FRAGMENT &&
+	if (ip->ip_len <= ifp->if_mtu ||
+	    (ifp->if_hwassist & (CSUM_FRAGMENT | CSUM_TCPSEG) &&
 	    ((ip->ip_off & IP_DF) == 0))) {
 		ip->ip_len = htons(ip->ip_len);
 		ip->ip_off = htons(ip->ip_off);
diff -Naur sys.dist/netinet/tcp_output.c sys/netinet/tcp_output.c
--- sys.dist/netinet/tcp_output.c	Thu Feb 23 13:14:34 2006
+++ sys/netinet/tcp_output.c	Thu Aug 31 16:50:33 2006
@@ -105,6 +105,12 @@
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, newreno, CTLFLAG_RW, &tcp_do_newreno,
 	0, "Enable NewReno Algorithms");
 
+#ifdef TCP_TSO
+int	tcp_enable_tso = 1;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, tso_enable, CTLFLAG_RW, &tcp_enable_tso,
+	0, "Enable TCP Segmentation Offload");
+#endif
+
 /*
  * Tcp output routine: figure out what should be sent and send it.
  */
@@ -130,6 +136,9 @@
 #if 0
 	int maxburst = TCP_MAXBURST;
 #endif
+#ifdef TCP_TSO
+	int tso_capable, use_tso;
+#endif
 #ifdef INET6
 	struct ip6_hdr *ip6 = NULL;
 	int isipv6;
@@ -139,6 +148,10 @@
 
 	INP_LOCK_ASSERT(tp->t_inpcb);
 
+#ifdef TCP_TSO
+	if (tcp_enable_tso)
+		use_tso = tso_capable = tcp_checktso(tp);
+#endif
 	/*
 	 * Determine length of data that should be transmitted,
 	 * and flags that will be used.
@@ -380,9 +393,22 @@
 	 * no longer contains the last data byte.
 	 */
 	if (len > tp->t_maxseg) {
-		len = tp->t_maxseg;
+#ifdef TCP_TSO
+		if (use_tso) {
+			/* Even out the transmissions */
+			len = ((ulmin(len, TCP_MAXWIN)/ tp->t_maxseg) *
+			    tp->t_maxseg);
+			if (len <= tp->t_maxseg)
+				use_tso = 0;
+                } else
+#endif
+			len = tp->t_maxseg;
 		sendalot = 1;
 	}
+#ifdef TCP_TSO
+	  else
+		use_tso = 0;
+#endif
 	if (sack_rxmit) {
 		if (SEQ_LT(p->rxmit + len, tp->snd_una + so->so_snd.sb_cc))
 			flags &= ~TH_FIN;
@@ -406,7 +432,7 @@
 	 *	- we need to retransmit
 	 */
 	if (len) {
-		if (len == tp->t_maxseg)
+		if (len >= tp->t_maxseg) /* TSO makes it possible to be greater */
 			goto send;
 		/*
 		 * NOTE! on localhost connections an 'ack' from the remote
@@ -703,6 +729,19 @@
 	 * Clear the FIN bit because we cut off the tail of
 	 * the segment.
 	 */
+#ifdef TCP_TSO
+	if (use_tso) {
+		if (len + optlen + ipoptlen > TCP_MAXWIN) {
+			/*
+			 * If there is still more to send,
+			 * don't close the connection.
+			 */
+			flags &= ~TH_FIN;
+			len = TCP_MAXWIN - optlen - ipoptlen;
+			sendalot = 1;
+		}
+	} else 
+#endif
 	if (len + optlen + ipoptlen > tp->t_maxopd) {
 		/*
 		 * If there is still more to send, don't close the connection.
@@ -944,6 +983,12 @@
 		/* IP version must be set here for ipv4/ipv6 checking later */
 		KASSERT(ip->ip_v == IPVERSION,
 		    ("%s: IP version incorrect: %d", __func__, ip->ip_v));
+#ifdef TCP_TSO
+		if (use_tso) {  /* Setup for TSO */
+			m->m_pkthdr.tso_mss = tp->t_maxseg;
+			m->m_pkthdr.csum_flags |= CSUM_TCPSEG;
+		}
+#endif
 	}
 
 	/*
@@ -1076,7 +1121,11 @@
 	 * Section 2. However the tcp hostcache migitates the problem
 	 * so it affects only the first tcp connection with a host.
 	 */
+#ifdef TCP_TSO
+	if (path_mtu_discovery && !use_tso)
+#else
 	if (path_mtu_discovery)
+#endif
 		ip->ip_off |= IP_DF;
 
 	error = ip_output(m, tp->t_inpcb->inp_options, NULL,
diff -Naur sys.dist/netinet/tcp_subr.c sys/netinet/tcp_subr.c
--- sys.dist/netinet/tcp_subr.c	Wed Aug  2 09:18:05 2006
+++ sys/netinet/tcp_subr.c	Thu Aug 31 16:54:31 2006
@@ -1643,6 +1643,41 @@
 }
 #endif /* INET6 */
 
+#ifdef TCP_TSO
+/*
+ * Find the interface for this tcpcb and determine
+ * if a TSO hardware assist is available.
+ */
+boolean_t
+tcp_checktso(tp)
+	struct tcpcb *tp;
+{
+	struct in_conninfo *inc;
+	struct route sro;
+	struct sockaddr_in *dst;
+	struct ifnet *ifp;
+
+	inc = &tp->t_inpcb->inp_inc;
+	bzero(&sro, sizeof(sro));
+	if (inc->inc_faddr.s_addr != INADDR_ANY) {
+		dst = (struct sockaddr_in *)&sro.ro_dst;
+		dst->sin_family = AF_INET;
+		dst->sin_len = sizeof(*dst);
+		dst->sin_addr = inc->inc_faddr;
+		rtalloc_ign(&sro, RTF_CLONING);
+	}
+	if (sro.ro_rt != NULL) {
+		ifp = sro.ro_rt->rt_ifp;
+		if (ifp->if_hwassist & CSUM_TCPSEG) {
+			RTFREE(sro.ro_rt);
+			return TRUE;
+		}
+		RTFREE(sro.ro_rt);
+	}
+	return FALSE;
+}
+#endif
+
 #ifdef IPSEC
 /* compute ESP/AH header size for TCP, including outer IP header. */
 size_t
diff -Naur sys.dist/netinet/tcp_var.h sys/netinet/tcp_var.h
--- sys.dist/netinet/tcp_var.h	Mon Jun 26 08:35:25 2006
+++ sys/netinet/tcp_var.h	Thu Aug 31 12:42:25 2006
@@ -510,6 +510,10 @@
 u_long	 tcp_maxmtu6(struct in_conninfo *);
 void	 tcp_mss(struct tcpcb *, int);
 int	 tcp_mssopt(struct in_conninfo *);
+#ifdef TCP_TSO
+boolean_t
+	tcp_checktso(struct tcpcb *);
+#endif
 struct inpcb *
 	 tcp_drop_syn_sent(struct inpcb *, int);
 struct inpcb *
diff -Naur sys.dist/sys/mbuf.h sys/sys/mbuf.h
--- sys.dist/sys/mbuf.h	Sun Jul 23 18:49:57 2006
+++ sys/sys/mbuf.h	Thu Aug 31 12:38:04 2006
@@ -110,6 +110,7 @@
 	/* variables for hardware checksum */
 	int	csum_flags;		/* flags regarding checksum */
 	int	csum_data;		/* data field used by csum routines */
+	int	tso_mss;		/* TSO segment size */
 	SLIST_HEAD(packet_tags, m_tag) tags; /* list of packet tags */
 };
 
@@ -215,6 +216,7 @@
 #define	CSUM_UDP		0x0004		/* will csum UDP */
 #define	CSUM_IP_FRAGS		0x0008		/* will csum IP fragments */
 #define	CSUM_FRAGMENT		0x0010		/* will do IP fragmentation */
+#define	CSUM_TCPSEG		0x0020		/* eligible for TCP segmentation */
 
 #define	CSUM_IP_CHECKED		0x0100		/* did csum IP */
 #define	CSUM_IP_VALID		0x0200		/*   ... the csum is valid */