Index: head/sys/netinet/tcp_input.c
===================================================================
--- head/sys/netinet/tcp_input.c
+++ head/sys/netinet/tcp_input.c
@@ -1674,10 +1674,19 @@
 		    (to.to_flags & TOF_SACKPERM) == 0)
 			tp->t_flags &= ~TF_SACK_PERMIT;
 		if (IS_FASTOPEN(tp->t_flags)) {
-			if (to.to_flags & TOF_FASTOPEN)
-				tcp_fastopen_update_cache(tp, to.to_mss,
+			if (to.to_flags & TOF_FASTOPEN) {
+				uint16_t mss;
+
+				if (to.to_flags & TOF_MSS)
+					mss = to.to_mss;
+				else
+					if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
+						mss = TCP6_MSS;
+					else
+						mss = TCP_MSS;
+				tcp_fastopen_update_cache(tp, mss,
 				    to.to_tfo_len, to.to_tfo_cookie);
-			else
+			} else
 				tcp_fastopen_disable_path(tp);
 		}
 	}
Index: head/sys/netinet/tcp_stacks/fastpath.c
===================================================================
--- head/sys/netinet/tcp_stacks/fastpath.c
+++ head/sys/netinet/tcp_stacks/fastpath.c
@@ -1763,10 +1763,19 @@
 		    (to.to_flags & TOF_SACKPERM) == 0)
 			tp->t_flags &= ~TF_SACK_PERMIT;
 		if (IS_FASTOPEN(tp->t_flags)) {
-			if (to.to_flags & TOF_FASTOPEN)
-				tcp_fastopen_update_cache(tp, to.to_mss,
+			if (to.to_flags & TOF_FASTOPEN) {
+				uint16_t mss;
+
+				if (to.to_flags & TOF_MSS)
+					mss = to.to_mss;
+				else
+					if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
+						mss = TCP6_MSS;
+					else
+						mss = TCP_MSS;
+				tcp_fastopen_update_cache(tp, mss,
 				    to.to_tfo_len, to.to_tfo_cookie);
-			else
+			} else
 				tcp_fastopen_disable_path(tp);
 		}
 	}
@@ -2220,10 +2229,19 @@
 		    (to.to_flags & TOF_SACKPERM) == 0)
 			tp->t_flags &= ~TF_SACK_PERMIT;
 		if (IS_FASTOPEN(tp->t_flags)) {
-			if (to.to_flags & TOF_FASTOPEN)
-				tcp_fastopen_update_cache(tp, to.to_mss,
+			if (to.to_flags & TOF_FASTOPEN) {
+				uint16_t mss;
+
+				if (to.to_flags & TOF_MSS)
+					mss = to.to_mss;
+				else
+					if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
+						mss = TCP6_MSS;
+					else
+						mss = TCP_MSS;
+				tcp_fastopen_update_cache(tp, mss,
 				    to.to_tfo_len, to.to_tfo_cookie);
-			else
+			} else
 				tcp_fastopen_disable_path(tp);
 		}
 	}
Index: head/sys/netinet/tcp_stacks/rack.c
===================================================================
--- head/sys/netinet/tcp_stacks/rack.c
+++ head/sys/netinet/tcp_stacks/rack.c
@@ -6657,10 +6657,19 @@
 			    (to.to_flags & TOF_SACKPERM) == 0)
 				tp->t_flags &= ~TF_SACK_PERMIT;
 			if (IS_FASTOPEN(tp->t_flags)) {
-				if (to.to_flags & TOF_FASTOPEN)
-					tcp_fastopen_update_cache(tp, to.to_mss,
+				if (to.to_flags & TOF_FASTOPEN) {
+					uint16_t mss;
+
+					if (to.to_flags & TOF_MSS)
+						mss = to.to_mss;
+					else
+						if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
+							mss = TCP6_MSS;
+						else
+							mss = TCP_MSS;
+					tcp_fastopen_update_cache(tp, mss,
 					    to.to_tfo_len, to.to_tfo_cookie);
-				else
+				} else
 					tcp_fastopen_disable_path(tp);
 			}
 		}