diff --git a/sys/netinet/sctp_input.c b/sys/netinet/sctp_input.c
index a1e79e41c1bf..a6f625d13a11 100644
--- a/sys/netinet/sctp_input.c
+++ b/sys/netinet/sctp_input.c
@@ -5684,11 +5684,18 @@ sctp_common_input_processing(struct mbuf **mm, int iphlen, int offset, int lengt
 			stcb = sctp_findassociation_addr(m, offset, src, dst,
 			    sh, ch, &inp, &net, vrf_id);
 #if defined(INET) || defined(INET6)
-			if ((net != NULL) &&
-			    (ch->chunk_type != SCTP_INITIATION) &&
-			    (port != 0)) {
+			if ((ch->chunk_type != SCTP_INITIATION) &&
+			    (net != NULL) && (net->port != port)) {
 				if (net->port == 0) {
-					sctp_pathmtu_adjustment(stcb, net->mtu - sizeof(struct udphdr));
+					/* UDP encapsulation turned on. */
+					net->mtu -= sizeof(struct udphdr);
+					if (stcb->asoc.smallest_mtu > net->mtu) {
+						sctp_pathmtu_adjustment(stcb, net->mtu);
+					}
+				} else if (port == 0) {
+					/* UDP encapsulation turned off. */
+					net->mtu += sizeof(struct udphdr);
+					/* XXX Update smallest_mtu */
 				}
 				net->port = port;
 			}
@@ -5717,11 +5724,18 @@ sctp_common_input_processing(struct mbuf **mm, int iphlen, int offset, int lengt
 	stcb = sctp_findassociation_addr(m, offset, src, dst,
 	    sh, ch, &inp, &net, vrf_id);
 #if defined(INET) || defined(INET6)
-	if ((net != NULL) &&
-	    (ch->chunk_type != SCTP_INITIATION) &&
-	    (port != 0)) {
+	if ((ch->chunk_type != SCTP_INITIATION) &&
+	    (net != NULL) && (net->port != port)) {
 		if (net->port == 0) {
-			sctp_pathmtu_adjustment(stcb, net->mtu - sizeof(struct udphdr));
+			/* UDP encapsulation turned on. */
+			net->mtu -= sizeof(struct udphdr);
+			if (stcb->asoc.smallest_mtu > net->mtu) {
+				sctp_pathmtu_adjustment(stcb, net->mtu);
+			}
+		} else if (port == 0) {
+			/* UDP encapsulation turned off. */
+			net->mtu += sizeof(struct udphdr);
+			/* XXX Update smallest_mtu */
 		}
 		net->port = port;
 	}
@@ -5831,11 +5845,18 @@ sctp_common_input_processing(struct mbuf **mm, int iphlen, int offset, int lengt
 			 */
 			inp = stcb->sctp_ep;
 #if defined(INET) || defined(INET6)
-			if ((net != NULL) &&
-			    (ch->chunk_type != SCTP_INITIATION) &&
-			    (port != 0)) {
+			if ((ch->chunk_type != SCTP_INITIATION) &&
+			    (net != NULL) && (net->port != port)) {
 				if (net->port == 0) {
-					sctp_pathmtu_adjustment(stcb, net->mtu - sizeof(struct udphdr));
+					/* UDP encapsulation turned on. */
+					net->mtu -= sizeof(struct udphdr);
+					if (stcb->asoc.smallest_mtu > net->mtu) {
+						sctp_pathmtu_adjustment(stcb, net->mtu);
+					}
+				} else if (port == 0) {
+					/* UDP encapsulation turned off. */
+					net->mtu += sizeof(struct udphdr);
+					/* XXX Update smallest_mtu */
 				}
 				net->port = port;
 			}