[mle] track remaining Link Request attempts when re-establishing links (#11160)

This commit updates `Router` to track the number of remaining Link
Request attempts during link re-establishment (when no Advertisement
is received from a router, and the router is aged out). This helps
simplify the code managing link re-establishment.

src/core/thread/mle.cpp

@@ -4614,6 +4614,15 @@ void Mle::DelayedSender::RemoveScheduledLinkRequest(const Router &aRouter)
     RemoveMatchingSchedules(kTypeLinkRequest, destination);
 }
 
+bool Mle::DelayedSender::HasAnyScheduledLinkRequest(const Router &aRouter) const
+{
+    Ip6::Address destination;
+
+    destination.SetToLinkLocalAddress(aRouter.GetExtAddress());
+
+    return HasMatchingSchedule(kTypeLinkRequest, destination);
+}
+
 void Mle::DelayedSender::ScheduleLinkAccept(const LinkAcceptInfo &aInfo, uint16_t aDelay)
 {
     Ip6::Address destination;

src/core/thread/mle.hpp

@@ -1142,11 +1142,12 @@ private:
         void ScheduleAdvertisement(const Ip6::Address &aDestination, uint16_t aDelay);
         void ScheduleMulticastDataResponse(uint16_t aDelay);
         void ScheduleLinkRequest(const Router &aRouter, uint16_t aDelay);
+        void RemoveScheduledLinkRequest(const Router &aRouter);
+        bool HasAnyScheduledLinkRequest(const Router &aRouter) const;
         void ScheduleLinkAccept(const LinkAcceptInfo &aInfo, uint16_t aDelay);
         void ScheduleDiscoveryResponse(const Ip6::Address          &aDestination,
                                        const DiscoveryResponseInfo &aInfo,
                                        uint16_t                     aDelay);
-        void RemoveScheduledLinkRequest(const Router &aRouter);
 #endif
         void RemoveScheduledChildUpdateRequestToParent(void);
 

src/core/thread/mle_router.cpp

@@ -1720,32 +1720,6 @@ void MleRouter::HandleTimeTick(void)
 
         age = TimerMilli::GetNow() - router.GetLastHeard();
 
-        if (router.IsStateValid() && (age >= kMaxNeighborAge))
-        {
-            bool sendLinkRequest = true;
-
-            // Once router age expires, we send Link Request every
-            // time tick (second), up to `kMaxTxCount`. Each rx is
-            // randomly delayed (one second window). After the last
-            // attempt, we wait for the "Link Accept" timeout
-            // (~3 seconds), before the router is removed.
-
-            if (!router.IsWaitingForLinkAccept())
-            {
-                LogInfo("No Adv from router 0x%04x - sending Link Request", router.GetRloc16());
-            }
-            else
-            {
-                sendLinkRequest = (age < kMaxNeighborAge + kMaxTxCount * TimeMilli::kOneSecondInMsec);
-            }
-
-            if (sendLinkRequest)
-            {
-                mDelayedSender.ScheduleLinkRequest(
-                    router, Random::NonCrypto::GetUint32InRange(0, kMaxLinkRequestDelayOnRouter));
-            }
-        }
-
 #if OPENTHREAD_CONFIG_PARENT_SEARCH_ENABLE
         router.DecrementParentReselectTimeout();
 
@@ -1755,6 +1729,28 @@ void MleRouter::HandleTimeTick(void)
         }
 #endif
 
+        if (router.IsStateValid() && (age >= kMaxNeighborAge))
+        {
+            // Once router age expires, we send Link Request every
+            // time tick (second), up to max attempts. Each rx is
+            // randomly delayed (one second window). After the last
+            // attempt, we wait for the "Link Accept" timeout
+            // (~3 seconds), before the router is removed.
+
+            if (!mDelayedSender.HasAnyScheduledLinkRequest(router) && !router.IsWaitingForLinkAccept())
+            {
+                LogInfo("No Adv from router 0x%04x - sending Link Request", router.GetRloc16());
+                router.SetLinkRequestAttemptsToMax();
+            }
+
+            if (router.HasRemainingLinkRequestAttempts())
+            {
+                router.DecrementLinkRequestAttempts();
+                mDelayedSender.ScheduleLinkRequest(
+                    router, Random::NonCrypto::GetUint32InRange(0, kMaxLinkRequestDelayOnRouter));
+            }
+        }
+
         if (router.IsWaitingForLinkAccept() && (router.DecrementLinkAcceptTimeout() == 0))
         {
             LogInfo("Router 0x%04x - Link Accept timeout expired", router.GetRloc16());

src/core/thread/mle_types.hpp

@@ -91,7 +91,9 @@ constexpr uint8_t kMaxRouteCost = 16; ///< Maximum path cost
 #endif
 
 constexpr uint8_t kMeshLocalPrefixContextId = 0; ///< Reserved 6lowpan context ID for Mesh Local Prefix
-constexpr uint8_t kLinkAcceptTimeout        = 3; ///< Timeout in seconds to rx Link Accept after Link Request tx.
+
+constexpr uint8_t kLinkRequestAttempts = 3; ///< Number of Link Request attempts when re-establishing link.
+constexpr uint8_t kLinkAcceptTimeout   = 3; ///< Timeout in seconds to rx Link Accept after Link Request tx.
 
 /**
  * Specifies parent reselect timeout duration in seconds used on FTD child devices.

src/core/thread/router.hpp

@@ -118,6 +118,27 @@ public:
      */
     uint8_t DecrementLinkAcceptTimeout(void) { return --mLinkAcceptTimeout; }
 
+    /**
+     * Sets the counter tracking the number of Link Request attempts during link re-establishment to its maximum value
+     * `Mle::kLinkRequestAttempts`.
+     */
+    void SetLinkRequestAttemptsToMax(void) { mLinkRequestAttempts = Mle::kLinkRequestAttempts; }
+
+    /**
+     * Indicates whether there are remaining Link Request attempts (during link re-establishment).
+     *
+     * @retval TRUE   There are remaining Link Request attempts.
+     * @retval FALSE  There are no more Link Request attempts (the counter is zero).
+     */
+    bool HasRemainingLinkRequestAttempts(void) const { return mLinkRequestAttempts > 0; }
+
+    /**
+     * Decrements the counter tracking the number of remaining Link Request attempts during link re-establishment.
+     *
+     * Caller MUST ensure the current counter is non-zero by checking `HasRemainingLinkRequestAttempts()`.
+     */
+    void DecrementLinkRequestAttempts(void) { mLinkRequestAttempts--; }
+
     /**
      * Gets the router ID of the next hop to this router.
      *
@@ -208,14 +229,16 @@ public:
 
 private:
     static_assert(Mle::kLinkAcceptTimeout < 4, "kLinkAcceptTimeout won't fit in mLinkAcceptTimeout (2-bit field)");
+    static_assert(Mle::kLinkRequestAttempts < 4, "kLinkRequestAttempts won't fit in mLinkRequestAttempts (2-bit field");
 
 #if OPENTHREAD_CONFIG_PARENT_SEARCH_ENABLE
     static_assert(Mle::kParentReselectTimeout <= (1U << 15) - 1,
                   "kParentReselectTimeout won't fit in mParentReselectTimeout (15-bit filed)");
 #endif
 
-    uint8_t mNextHop;               // The next hop towards this router
-    uint8_t mLinkAcceptTimeout : 2; // Timeout (in seconds) after sending Link Request waiting for Link Accept
+    uint8_t mNextHop;                 // The next hop towards this router
+    uint8_t mLinkRequestAttempts : 2; // Number of Link Request attempts
+    uint8_t mLinkAcceptTimeout : 2;   // Timeout (in seconds) after sending Link Request waiting for Link Accept
 #if !OPENTHREAD_CONFIG_MLE_LONG_ROUTES_ENABLE
     uint8_t mCost : 4; // The cost to this router via neighbor router
 #else