diff --git a/include/mtp_data.h b/include/mtp_data.h
index 8c34f20..5168eb1 100644
--- a/include/mtp_data.h
+++ b/include/mtp_data.h
@@ -46,6 +46,8 @@ struct mtp_link {
 	int running;
 	int sccp_up;
 
+	int last_sls;
+
 	/* misc data */
 	uint8_t test_ptrn[14];
 
diff --git a/src/bsc_sccp.c b/src/bsc_sccp.c
index 69e2262..ea13c43 100644
--- a/src/bsc_sccp.c
+++ b/src/bsc_sccp.c
@@ -86,7 +86,7 @@ unsigned int sls_for_src_ref(struct sccp_source_reference *ref)
 
 	con = find_con_by_src_ref(ref);
 	if (!con)
-		return 13;
+		return -1;
 	return con->sls;
 }
 
diff --git a/src/main.c b/src/main.c
index c922474..a4e502c 100644
--- a/src/main.c
+++ b/src/main.c
@@ -224,7 +224,7 @@ static void bsc_reset_timeout(void *_data)
 	}
 
 	++bsc->reset_count;
-	mtp_link_submit_sccp_data(bsc->link.the_link, 13, msg->l2h, msgb_l2len(msg));
+	mtp_link_submit_sccp_data(bsc->link.the_link, -1, msg->l2h, msgb_l2len(msg));
 	msgb_free(msg);
 	bsc_schedule_timer(&bsc->reset_timeout, 20, 0);
 }
@@ -351,7 +351,7 @@ static void handle_rlsd(struct sccp_connection_released *rlsd, int from_msc)
 				 &rlsd->source_local_reference);
 		}
 	} else {
-		unsigned int sls = 13;
+		unsigned int sls = -1;
 		con = find_con_by_src_dest_ref(&rlsd->source_local_reference,
 					       &rlsd->destination_local_reference);
 		if (con) {
diff --git a/src/msc_conn.c b/src/msc_conn.c
index 5d1b329..62459ec 100644
--- a/src/msc_conn.c
+++ b/src/msc_conn.c
@@ -168,7 +168,7 @@ static int ipaccess_a_fd_cb(struct bsc_fd *bfd)
 
 		/* we can not forward it right now */
 		if (bsc->forward_only && link->sccp_up) {
-			if (send_or_queue_bsc_msg(link, 13, msg) != 1)
+			if (send_or_queue_bsc_msg(link, -1, msg) != 1)
 				msgb_free(msg);
 			return 0;
 		}
diff --git a/src/mtp_layer3.c b/src/mtp_layer3.c
index 91d3f07..e756440 100644
--- a/src/mtp_layer3.c
+++ b/src/mtp_layer3.c
@@ -510,6 +510,11 @@ int mtp_link_submit_sccp_data(struct mtp_link *link, int sls, const uint8_t *dat
 		return -1;
 	}
 
+	if (sls == -1) {
+		sls = link->last_sls;
+		link->last_sls = (link->last_sls + 1) % 16;
+	}
+
 	return mtp_int_submit(link, link->sccp_opc, sls, MTP_SI_MNT_SCCP, data, length);
 }