TSO broken with jumbo MTU

[Ben Hutchings]

This is the fix/workaround I used:

--- a/netinet/tcp_output.c
+++ b/netinet/tcp_output.c
@@ -1062,9 +1062,7 @@
 	 * The TCP pseudo header checksum is always provided.
 	 * XXX: Fixme: This is currently not the case for IPv6.
 	 */
-	if (tso) {
-		KASSERT(len > tp->t_maxopd - optlen,
-		    ("%s: len <= tso_segsz", __func__));
+	if (tso && len > tp->t_maxopd - optlen) {
 		m->m_pkthdr.csum_flags |= CSUM_TSO;
 		m->m_pkthdr.tso_segsz = tp->t_maxopd - optlen;
 	}
--- END ---

But the correct thing to do may be to change the calculation of t_maxopd
(untested):

--- a/netinet/tcp_input.c
+++ b/netinet/tcp_input.c
@@ -3087,7 +3087,7 @@
 tcp_mss_update(struct tcpcb *tp, int offer,
     struct hc_metrics_lite *metricptr, int *mtuflags)
 {
-	int mss;
+	int mss, ts_len;
 	u_long maxmtu;
 	struct inpcb *inp = tp->t_inpcb;
 	struct hc_metrics_lite metrics;
@@ -3212,22 +3212,17 @@
 	mss = max(mss, 64);
 
 	/*
-	 * maxopd stores the maximum length of data AND options
-	 * in a segment; maxseg is the amount of data in a normal
-	 * segment.  We need to store this value (maxopd) apart
-	 * from maxseg, because now every segment carries options
-	 * and thus we normally have somewhat less data in segments.
-	 */
-	tp->t_maxopd = mss;
-
-	/*
 	 * origoffer==-1 indicates that no segments were received yet.
 	 * In this case we just guess.
 	 */
 	if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
 	    (origoffer == -1 ||
 	     (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP))
-		mss -= TCPOLEN_TSTAMP_APPA;
+		ts_len = TCPOLEN_TSTAMP_APPA;
+	else
+		ts_len = 0;
+
+	mss -= ts_len;
 
 #if	(MCLBYTES & (MCLBYTES - 1)) == 0
 	if (mss > MCLBYTES)
@@ -3237,6 +3232,15 @@
 		mss = mss / MCLBYTES * MCLBYTES;
 #endif
 	tp->t_maxseg = mss;
+
+	/*
+	 * maxopd stores the maximum length of data AND options
+	 * in a segment; maxseg is the amount of data in a normal
+	 * segment.  We need to store this value (maxopd) apart
+	 * from maxseg, because now every segment carries options
+	 * and thus we normally have somewhat less data in segments.
+	 */
+	tp->t_maxopd = mss + ts_len;
 }
 
 void
--- END ---

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.