diff --git a/sys/dev/cxgbe/tom/t4_cpl_io.c b/sys/dev/cxgbe/tom/t4_cpl_io.c
index f1e2dfac372a..3cee10ae06a1 100644
--- a/sys/dev/cxgbe/tom/t4_cpl_io.c
+++ b/sys/dev/cxgbe/tom/t4_cpl_io.c
@@ -44,6 +44,7 @@ __FBSDID("$FreeBSD$");
 #include <netinet/in.h>
 #include <netinet/in_pcb.h>
 #include <netinet/ip.h>
+#include <netinet/ip6.h>
 #include <netinet/tcp_var.h>
 #define TCPSTATES
 #include <netinet/tcp_fsm.h>
@@ -236,11 +237,20 @@ static void
 assign_rxopt(struct tcpcb *tp, unsigned int opt)
 {
 	struct toepcb *toep = tp->t_toe;
+	struct inpcb *inp = tp->t_inpcb;
 	struct adapter *sc = td_adapter(toep->td);
+	int n;
 
-	INP_LOCK_ASSERT(tp->t_inpcb);
+	INP_LOCK_ASSERT(inp);
 
-	tp->t_maxseg = tp->t_maxopd = sc->params.mtus[G_TCPOPT_MSS(opt)] - 40;
+	if (inp->inp_inc.inc_flags & INC_ISIPV6)
+		n = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
+	else
+		n = sizeof(struct ip) + sizeof(struct tcphdr);
+	tp->t_maxseg = tp->t_maxopd = sc->params.mtus[G_TCPOPT_MSS(opt)] - n;
+
+	CTR4(KTR_CXGBE, "%s: tid %d, mtu_idx %u (%u)", __func__, toep->tid,
+	    G_TCPOPT_MSS(opt), sc->params.mtus[G_TCPOPT_MSS(opt)]);
 
 	if (G_TCPOPT_TSTAMP(opt)) {
 		tp->t_flags |= TF_RCVD_TSTMP;	/* timestamps ok */