[mle] limit number of child-router links a REED / FED maintains (#3358)

src/core/openthread-core-default-config.h

@@ -1662,6 +1662,16 @@
 #define OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN 10
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_MLE_CHILD_ROUTER_LINKS
+ *
+ * Specifies the desired number of router links that a REED / FED attempts to maintain.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_MLE_CHILD_ROUTER_LINKS
+#define OPENTHREAD_CONFIG_MLE_CHILD_ROUTER_LINKS 3
+#endif
+
 /**
  * @def OPENTHREAD_CONFIG_ENABLE_DEBUG_UART
  *

src/core/thread/mle_router.cpp

@@ -1352,7 +1352,8 @@ otError MleRouter::HandleAdvertisement(const Message &aMessage, const Ip6::Messa
             }
         }
         else if (IsFullThreadDevice() && (router->GetState() != Neighbor::kStateValid) &&
-                 (router->GetState() != Neighbor::kStateLinkRequest))
+                 (router->GetState() != Neighbor::kStateLinkRequest) &&
+                 (mRouterTable.GetActiveLinkCount() < OPENTHREAD_CONFIG_MLE_CHILD_ROUTER_LINKS))
         {
             router->SetExtAddress(macAddr);
             router->GetLinkInfo().Clear();

src/core/thread/router_table.cpp

@@ -335,6 +335,28 @@ void RouterTable::RemoveNeighbor(Router &aRouter)
     }
 }
 
+uint8_t RouterTable::GetActiveLinkCount(void) const
+{
+    uint8_t activeLinks = 0;
+
+    for (int i = 0; i < Mle::kMaxRouters; i++)
+    {
+        const Router &cur = mRouters[i];
+
+        if (cur.GetRloc16() == 0xffff)
+        {
+            break;
+        }
+
+        if (cur.GetState() == Neighbor::kStateValid)
+        {
+            activeLinks++;
+        }
+    }
+
+    return activeLinks;
+}
+
 Router *RouterTable::GetNeighbor(uint16_t aRloc16)
 {
     Router *router = NULL;

src/core/thread/router_table.hpp

@@ -176,6 +176,14 @@ public:
      */
     uint8_t GetActiveRouterCount(void) const { return mActiveRouterCount; }
 
+    /**
+     * This method returns the number of active links with neighboring routers.
+     *
+     * @returns The number of active links with neighboring routers.
+     *
+     */
+    uint8_t GetActiveLinkCount(void) const;
+
     /**
      * This method returns the leader in the Thread network.
      *