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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);

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