diff --git a/src/include/netif/ppp/pppoe.h b/src/include/netif/ppp/pppoe.h
index 802a62d1..b1dd2fd9 100644
--- a/src/include/netif/ppp/pppoe.h
+++ b/src/include/netif/ppp/pppoe.h
@@ -167,6 +167,10 @@ ppp_pcb *pppoe_create(struct netif *pppif,
        const char *service_name, const char *concentrator_name,
        ppp_link_status_cb_fn link_status_cb, void *ctx_cb);
 
+/*
+ * Functions called from lwIP
+ * DO NOT CALL FROM lwIP USER APPLICATION.
+ */
 void pppoe_disc_input(struct netif *netif, struct pbuf *p);
 void pppoe_data_input(struct netif *netif, struct pbuf *p);
 
diff --git a/src/include/netif/ppp/pppos.h b/src/include/netif/ppp/pppos.h
index 62d11acf..3c1298ac 100644
--- a/src/include/netif/ppp/pppos.h
+++ b/src/include/netif/ppp/pppos.h
@@ -97,6 +97,11 @@ ppp_pcb *pppos_create(struct netif *pppif, sio_fd_t fd,
 void pppos_input(ppp_pcb *ppp, u_char* data, int len);
 
 
+/*
+ * Functions called from PPP CORE
+ *
+ * You may use them if you REALLY know what you are doing.
+ */
 void pppos_accm_out_config(pppos_pcb *pppos, u32_t accm);
 void pppos_accm_in_config(pppos_pcb *pppos, u32_t accm);
 sio_fd_t pppos_get_fd(pppos_pcb *pppos);