[mle] add retransmissions to MLE Link Request on neighbor timeout (#2336)

src/core/thread/mle_constants.hpp

@@ -64,6 +64,7 @@ enum
     kParentResponseMaxDelayRouters = 500,   ///< Maximum delay for response for Parent Request sent to routers only
     kParentResponseMaxDelayAll     = 1000,  ///< Maximum delay for response for Parent Request sent to all devices
     kUnicastRetransmissionDelay    = 1000,  ///< Base delay before retransmitting an MLE unicast.
+    kMaxTransmissionCount          = 3,     ///< Maximum number of times an MLE message may be transmitted.
     kMaxResponseDelay              = 1000,  ///< Maximum delay before responding to a multicast request
     kMaxChildIdRequestTimeout      = 5000,  ///< Maximum delay for receiving a Child ID Request
     kMaxChildUpdateResponseTimeout = 2000,  ///< Maximum delay for receiving a Child Update Response

src/core/thread/mle_router.cpp

@@ -1935,16 +1935,18 @@ void MleRouter::HandleStateUpdateTimer(void)
 
 #else
 
-            if (age >= TimerMilli::SecToMsec(kMaxNeighborAge) &&
-                age < TimerMilli::SecToMsec(kMaxNeighborAge) + kStateUpdatePeriod)
+            if (age >= TimerMilli::SecToMsec(kMaxNeighborAge))
             {
-                otLogInfoMle(GetInstance(), "Router timeout expired");
-                SendLinkRequest(&router);
-            }
-            else if (age >= TimerMilli::SecToMsec(kMaxNeighborAge) + kMaxLinkRequestTimeout)
-            {
-                RemoveNeighbor(router);
-                continue;
+                if (age < TimerMilli::SecToMsec(kMaxNeighborAge) + kMaxTransmissionCount * kUnicastRetransmissionDelay)
+                {
+                    otLogInfoMle(GetInstance(), "Router timeout expired");
+                    SendLinkRequest(&router);
+                }
+                else
+                {
+                    RemoveNeighbor(router);
+                    continue;
+                }
             }
 
 #endif