Add some comments and remove a FIXME

core/federated/RTI/rti_common.c

@@ -490,7 +490,6 @@ tag_t get_DNET_candidate(tag_t received_tag, tag_t minimum_delay) {
   return result;
 }
 
-// FIXME: It should be static because it's used only in this file. Remove from the header file.
 void downstream_next_event_tag_if_needed(scheduling_node_t* node, uint16_t new_NET_source_federate_id) {
   if (is_in_zero_delay_cycle(node)) {
     return;
@@ -526,7 +525,8 @@ void downstream_next_event_tag_if_needed(scheduling_node_t* node, uint16_t new_N
     }
   }
   if (DNET.time < start_time) {
-    // DNET is NEVER.
+    // DNET with the time smaller than the start time acts as the same as DNET of the NEVER tag.
+    // Thus, set DNET as NEVER_TAG to prevent sending unnecessary DNETs.
     DNET = NEVER_TAG;
   }
 

core/federated/RTI/rti_common.h

@@ -286,7 +286,8 @@ void update_min_delays_upstream(scheduling_node_t* node);
 void update_all_downstreams(scheduling_node_t* node);
 
 /**
- * Subtract the tag a from the tag b.
+ * Find the tag g that is the latest tag that satisfies lf_tag_add(g, minimum_delay) < received_tag.
+ * This function behaves like the tag subtraction, received_tag - minimum_delay.
  * minimum_delay cannot be NEVER.
  * @param received_tag
  * @param minimum_delay
@@ -300,8 +301,10 @@ tag_t get_DNET_candidate(tag_t received_tag, tag_t minimum_delay);
 void notify_downstream_next_event_tag(scheduling_node_t* e, tag_t tag);
 
 /**
- * @param node
- * @param new_NET
+ * @param node The target node that may receive a new DNET.
+ * @param new_NET_source_federate_id The ID of the federate that sends a new NET. If this federate's new NET does not
+ * change the DNET value, we can exit this function immediately. If it does, we have to look up the target federate's
+ * downstream federates to compute the exact new DNET value.
  */
 void downstream_next_event_tag_if_needed(scheduling_node_t* node, uint16_t new_NET_source_federate_id);
 

core/federated/federate.c

@@ -1468,7 +1468,6 @@ static void handle_stop_request_message() {
 
 /**
  * Handle a downstream next event tag (DNET) message from the RTI.
- * FIXME: Use this tag to eliminate unncessary LTC or NET messages.
  */
 static void handle_downstream_next_event_tag() {
   size_t bytes_to_read = sizeof(instant_t) + sizeof(microstep_t);
@@ -2412,7 +2411,9 @@ tag_t lf_send_next_event_tag(environment_t* env, tag_t tag, bool wait_for_reply)
         send_tag(MSG_TYPE_NEXT_EVENT_TAG, tag);
         _fed.last_sent_NET = tag;
         _fed.last_skipped_NET = NEVER_TAG;
-        LF_PRINT_LOG("Sent next event tag (NET) " PRINTF_TAG " to RTI.", tag.time - start_time, tag.microstep);
+        LF_PRINT_LOG("Sent the last skipped next event tag (NET) " PRINTF_TAG
+                     " to RTI based on the last DNET " PRINTF_TAG ".",
+                     _fed.last_DNET - start_time, _fed.last_DNET.microstep, tag.time - start_time, tag.microstep);
       }
       return _fed.last_TAG;
     }
@@ -2442,7 +2443,7 @@ tag_t lf_send_next_event_tag(environment_t* env, tag_t tag, bool wait_for_reply)
         LF_PRINT_LOG("Sent next event tag (NET) " PRINTF_TAG " to RTI.", tag.time - start_time, tag.microstep);
       } else {
         _fed.last_skipped_NET = tag;
-        LF_PRINT_LOG("Skip next event tag (NET) " PRINTF_TAG " to RTI.", tag.time - start_time, tag.microstep);
+        LF_PRINT_LOG("Skip sending next event tag (NET) " PRINTF_TAG " to RTI.", tag.time - start_time, tag.microstep);
       }
 
       if (!wait_for_reply) {

include/core/federated/federate.h

@@ -164,17 +164,17 @@ typedef struct federate_instance_t {
   tag_t last_sent_LTC;
 
   /**
-   * A record of the most recently sent NET (next event tag) message.
+   * A record of the most recently sent NET (next event tag) signal.
    */
   tag_t last_sent_NET;
 
   /**
-   * A record of the most recently skipped NET (next event tag) message.
+   * A record of the most recently skipped NET (next event tag) signal.
    */
   tag_t last_skipped_NET;
 
   /**
-   *
+   * A record of the most recent DNET (downstream next event tag) signal.
    */
   tag_t last_DNET;