Index: head/sys/net/if_var.h
===================================================================
--- head/sys/net/if_var.h
+++ head/sys/net/if_var.h
@@ -243,11 +243,12 @@
 	 * count limit does not apply. If all three fields are zero,
 	 * there is no TSO limit.
 	 *
-	 * NOTE: The TSO limits only apply to the data payload part of
-	 * a TCP/IP packet. That means there is no need to subtract
-	 * space for ethernet-, vlan-, IP- or TCP- headers from the
-	 * TSO limits unless the hardware driver in question requires
-	 * so.
+	 * NOTE: The TSO limits should reflect the values used in the
+	 * BUSDMA tag a network adapter is using to load a mbuf chain
+	 * for transmission. The TCP/IP network stack will subtract
+	 * space for all linklevel and protocol level headers and
+	 * ensure that the full mbuf chain passed to the network
+	 * adapter fits within the given limits.
 	 */
 	u_int	if_hw_tsomax;		/* TSO maximum size in bytes */
 	u_int	if_hw_tsomaxsegcount;	/* TSO maximum segment count */
Index: head/sys/netinet/tcp_output.c
===================================================================
--- head/sys/netinet/tcp_output.c
+++ head/sys/netinet/tcp_output.c
@@ -811,7 +811,8 @@
 			 */
 			if (if_hw_tsomax != 0) {
 				/* compute maximum TSO length */
-				max_len = (if_hw_tsomax - hdrlen);
+				max_len = (if_hw_tsomax - hdrlen -
+				    max_linkhdr);
 				if (max_len <= 0) {
 					len = 0;
 				} else if (len > max_len) {
@@ -826,6 +827,15 @@
 			 */
 			if (if_hw_tsomaxsegcount != 0 &&
 			    if_hw_tsomaxsegsize != 0) {
+				/*
+				 * Subtract one segment for the LINK
+				 * and TCP/IP headers mbuf that will
+				 * be prepended to this mbuf chain
+				 * after the code in this section
+				 * limits the number of mbufs in the
+				 * chain to if_hw_tsomaxsegcount.
+				 */
+				if_hw_tsomaxsegcount -= 1;
 				max_len = 0;
 				mb = sbsndmbuf(&so->so_snd, off, &moff);