[backbone-router] use TimeTicker directly to delay registration (#9483)

This commit update `BackboneRouter::Local` to be a `TimeTicker`
receiver directly instead of using the `MleRouter for this. The
`TimeTicker` callback is used to delay registration by counting down
`mRegistrationTimeout`.

src/core/backbone_router/bbr_local.cpp

@@ -54,6 +54,7 @@ Local::Local(Instance &aInstance)
     , mState(kStateDisabled)
     , mMlrTimeout(kDefaultMlrTimeout)
     , mReregistrationDelay(kDefaultRegistrationDelay)
+    , mRegistrationTimeout(0)
     , mSequenceNumber(Random::NonCrypto::GetUint8() % 127)
     , mRegistrationJitter(kDefaultRegistrationJitter)
     , mIsServiceAdded(false)
@@ -252,16 +253,15 @@ void Local::HandleBackboneRouterPrimaryUpdate(Leader::State aState, const Config
     // Wait some jitter before trying to Register.
     if (aConfig.mServer16 == Mac::kShortAddrInvalid)
     {
-        uint8_t delay = 1;
+        mRegistrationTimeout = 1;
 
         if (!Get<Mle::MleRouter>().IsLeader())
         {
-            delay += Random::NonCrypto::GetUint8InRange(0, mRegistrationJitter < 255 ? mRegistrationJitter + 1
-                                                                                     : mRegistrationJitter);
+            mRegistrationTimeout +=
+                Random::NonCrypto::GetUint16InRange(0, static_cast<uint16_t>(mRegistrationJitter) + 1);
         }
 
-        // Here uses the timer resource in Mle.
-        Get<Mle::MleRouter>().SetBackboneRouterRegistrationDelay(delay);
+        Get<TimeTicker>().RegisterReceiver(TimeTicker::kBbrLocal);
     }
     else if (aConfig.mServer16 != Get<Mle::MleRouter>().GetRloc16())
     {
@@ -287,6 +287,29 @@ exit:
     return;
 }
 
+void Local::HandleTimeTick(void)
+{
+    // Delay registration when `GetRouterSelectionJitterTimeout()` is non-zero,
+    // which indicates device may soon switch its role (e.g., REED to router).
+    VerifyOrExit(Get<Mle::MleRouter>().GetRouterSelectionJitterTimeout() == 0);
+
+    if (mRegistrationTimeout > 0)
+    {
+        mRegistrationTimeout--;
+
+        if (mRegistrationTimeout == 0)
+        {
+            IgnoreError(AddService(kDecideBasedOnState));
+        }
+    }
+
+exit:
+    if (mRegistrationTimeout == 0)
+    {
+        Get<TimeTicker>().UnregisterReceiver(TimeTicker::kBbrLocal);
+    }
+}
+
 Error Local::GetDomainPrefix(NetworkData::OnMeshPrefixConfig &aConfig)
 {
     Error error = kErrorNone;

src/core/backbone_router/bbr_local.hpp

@@ -59,6 +59,7 @@
 #include "common/locator.hpp"
 #include "common/log.hpp"
 #include "common/non_copyable.hpp"
+#include "common/time_ticker.hpp"
 #include "net/netif.hpp"
 #include "thread/network_data.hpp"
 
@@ -72,6 +73,8 @@ namespace BackboneRouter {
  */
 class Local : public InstanceLocator, private NonCopyable
 {
+    friend class ot::TimeTicker;
+
 public:
     /**
      * Represents Backbone Router state.
@@ -278,6 +281,7 @@ public:
 private:
     void SetState(State aState);
     void RemoveService(void);
+    void HandleTimeTick(void);
     void AddDomainPrefixToNetworkData(void);
     void RemoveDomainPrefixFromNetworkData(void);
     void SequenceNumberIncrease(void);
@@ -292,6 +296,7 @@ private:
     State    mState;
     uint32_t mMlrTimeout;
     uint16_t mReregistrationDelay;
+    uint16_t mRegistrationTimeout;
     uint8_t  mSequenceNumber;
     uint8_t  mRegistrationJitter;
 

src/core/common/time_ticker.cpp

@@ -127,6 +127,13 @@ void TimeTicker::HandleTimer(void)
     {
         Get<Ip6::Mpl>().HandleTimeTick();
     }
+
+#if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
+    if (mReceivers & Mask(kBbrLocal))
+    {
+        Get<BackboneRouter::Local>().HandleTimeTick();
+    }
+#endif
 }
 
 } // namespace ot

src/core/common/time_ticker.hpp

@@ -73,6 +73,7 @@ public:
         kMlrManager,             ///< `MlrManager`
         kNetworkDataNotifier,    ///< `NetworkData::Notifier`
         kIp6Mpl,                 ///< `Ip6::Mpl`
+        kBbrLocal,               ///< `BackboneRouter::Local`
 
         kNumReceivers, ///< Number of receivers.
     };

src/core/thread/mle_router.cpp

@@ -86,9 +86,6 @@ MleRouter::MleRouter(Instance &aInstance)
     , mRouterSelectionJitterTimeout(0)
     , mChildRouterLinks(kChildRouterLinks)
     , mParentPriority(kParentPriorityUnspecified)
-#if OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
-    , mBackboneRouterRegistrationDelay(0)
-#endif
 #if OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE
     , mMaxChildIpAddresses(0)
 #endif
@@ -1541,19 +1538,6 @@ void MleRouter::HandleTimeTick(void)
             routerStateUpdate = true;
         }
     }
-#if OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
-    // Delay register only when `mRouterSelectionJitterTimeout` is 0,
-    // that is, when the device has decided to stay as REED or Router.
-    else if (mBackboneRouterRegistrationDelay > 0)
-    {
-        mBackboneRouterRegistrationDelay--;
-
-        if (mBackboneRouterRegistrationDelay == 0)
-        {
-            IgnoreError(Get<BackboneRouter::Local>().AddService(BackboneRouter::Local::kDecideBasedOnState));
-        }
-    }
-#endif
 
     switch (mRole)
     {

src/core/thread/mle_router.hpp

@@ -522,16 +522,6 @@ public:
     Error SendTimeSync(void);
 #endif
 
-#if OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
-    /**
-     * Sets the delay before registering Backbone Router service.
-     *
-     * @param[in]  aDelay  The delay before registering Backbone Router service.
-     *
-     */
-    void SetBackboneRouterRegistrationDelay(uint8_t aDelay) { mBackboneRouterRegistrationDelay = aDelay; }
-#endif
-
     /**
      * Gets the maximum number of IP addresses that each MTD child may register with this device as parent.
      *
@@ -743,9 +733,6 @@ private:
     uint8_t mChildRouterLinks;
 
     int8_t mParentPriority; ///< The assigned parent priority value, -2 means not assigned.
-#if OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
-    uint8_t mBackboneRouterRegistrationDelay; ///< Delay before registering Backbone Router service.
-#endif
 #if OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE
     uint8_t mMaxChildIpAddresses;
 #endif