From e7385ab2c0315cff02771088ee394f88ad152154 Mon Sep 17 00:00:00 2001
From: Simon Duquennoy <simon.duquennoy@gmail.com>
Date: Thu, 29 Jun 2017 09:25:21 +0200
Subject: [PATCH] rpl-lite parent selection: if the preferred parent is still
 best but no longer fresh, stick to it and use urgent probing to get it fresh
 again

---
 core/net/rpl-lite/rpl-neighbor.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/core/net/rpl-lite/rpl-neighbor.c b/core/net/rpl-lite/rpl-neighbor.c
index c4b28c0e6..742e2397c 100644
--- a/core/net/rpl-lite/rpl-neighbor.c
+++ b/core/net/rpl-lite/rpl-neighbor.c
@@ -336,6 +336,7 @@ rpl_neighbor_select_best(void)
 #if RPL_WITH_PROBING
   if(best != NULL) {
     if(rpl_neighbor_is_fresh(best)) {
+      /* Return best if it is fresh */
       return best;
     } else {
       rpl_nbr_t *best_fresh;
@@ -350,6 +351,13 @@ rpl_neighbor_select_best(void)
         rpl_schedule_probing();
       }
 
+      /* The best is our preferred parent. It is not fresh but used to be,
+      else we would not have selected it in the first place. Stick to it
+      for a little while and rely on urgent probing to make a call. */
+      if(best == curr_instance.dag.preferred_parent) {
+        return best;
+      }
+
       /* Look for the best fresh parent. */
       best_fresh = best_parent(1);
       if(best_fresh == NULL) {
@@ -367,6 +375,7 @@ rpl_neighbor_select_best(void)
       }
     }
   } else {
+    /* No acceptable parent */
     return NULL;
   }
 #else /* RPL_WITH_PROBING */