diff --git a/lib/drivers/oskittcp/oskittcp/tcp_input.c b/lib/drivers/oskittcp/oskittcp/tcp_input.c
index 175cea40877..1811476e9cf 100644
--- a/lib/drivers/oskittcp/oskittcp/tcp_input.c
+++ b/lib/drivers/oskittcp/oskittcp/tcp_input.c
@@ -1961,7 +1961,14 @@ tcp_mss(tp, offer)
 
 	inp = tp->t_inpcb;
 	if ((rt = tcp_rtlookup(inp)) == NULL) {
+#ifndef __REACTOS__
 		tp->t_maxopd = tp->t_maxseg = tcp_mssdflt;
+#else
+		if (offer < tcp_mssdflt)
+			tp->t_maxopd = tp->t_maxseg = tcp_mssdflt;
+		else
+			tp->t_maxopd = tp->t_maxseg = min(offer, tcp_mssopt(tp));
+#endif
 		return;
 	}
 #ifndef __REACTOS__
@@ -2116,15 +2123,20 @@ int
 tcp_mssopt(tp)
 	struct tcpcb *tp;
 {
+#ifndef __REACTOS__
 	struct rtentry *rt;
 
 	rt = tcp_rtlookup(tp->t_inpcb);
 	if (rt == NULL)
 		return tcp_mssdflt;
-#ifndef __REACTOS__
+
 	return rt->rt_ifp->if_mtu - sizeof(struct tcpiphdr);
 #else
-	return tcp_mssdflt;
+	struct ifaddr *ifa = ifa_ifwithnet((struct sockaddr *)&tp->t_inpcb->inp_faddr);
+	if (ifa == NULL)
+		return tcp_mssdflt;
+
+	return ifa->ifa_mtu - sizeof(struct tcpiphdr);
 #endif
 }
 #endif /* TUBA_INCLUDE */