[mle] add wait time for resetting attach backoff interval after attach (#6202)

This commit adds a new behavior related to MLE attach backoff feature.
It adds a configurable delay wait interval before resetting the attach
backoff interval to its minimum value upon a successful attach.
`OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_DELAY_TO_RESET_BACKOFF_INTERVAL`
specifies the delay interval.

If it is set to zero then the device resets its backoff attach
interval immediately after a successful attach. With a non-zero value,
if after a successful attach, the device happens to detach within the
delay interval, the reattach process resumes with the previous backoff
interval (as if the attach did not happen).

This behavior is helpful in the situation where a battery-powered
device has poor link quality to its parent and therefore attaches and
detaches frequently from its parent.  Using a non-zero wait interval
ensures that the attach backoff does not reset on each attach and that
the device does not drain its battery quickly trying to re-attach too
frequently.

src/core/config/mle.h

@@ -163,6 +163,26 @@
 #define OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_JITTER_INTERVAL 2000
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_DELAY_TO_RESET_BACKOFF_INTERVAL
+ *
+ * Specifies the delay wait interval (in milliseconds) used by attach backoff feature after a successful attach before
+ * it resets the current backoff interval back to the minimum value.
+ *
+ * If it is set to zero then the device resets its backoff attach interval immediately after a successful attach. With
+ * a non-zero value, if after a successful attach, the device happens to detach within the delay interval, the reattach
+ *  process resumes with the previous backoff interval (as if the attach did not happen).
+ *
+ * This behavior is helpful in the situation where a battery-powered device has poor link quality to its parent and
+ * therefore attaches and detaches frequently from the parent.  Using a non-zero wait interval ensures that the attach
+ * backoff interval does not reset on each attach and that the device does not drain its battery quickly trying to
+ * re-attach too frequently.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_DELAY_TO_RESET_BACKOFF_INTERVAL
+#define OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_DELAY_TO_RESET_BACKOFF_INTERVAL 20000
+#endif
+
 /**
  * @def OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT
  *

src/core/thread/mle.cpp

@@ -502,6 +502,11 @@ otError Mle::BecomeChild(AttachMode aMode)
     VerifyOrExit(!IsDisabled(), error = OT_ERROR_INVALID_STATE);
     VerifyOrExit(!IsAttaching(), error = OT_ERROR_BUSY);
 
+    if (!IsDetached())
+    {
+        mAttachCounter = 0;
+    }
+
     if (mReattachState == kReattachStart)
     {
         if (Get<MeshCoP::ActiveDataset>().Restore() == OT_ERROR_NONE)
@@ -652,8 +657,7 @@ void Mle::SetStateChild(uint16_t aRloc16)
     SetRloc16(aRloc16);
     SetRole(kRoleChild);
     SetAttachState(kAttachStateIdle);
-    mAttachTimer.Stop();
-    mAttachCounter       = 0;
+    mAttachTimer.Start(kAttachBackoffDelayToResetCounter);
     mReattachState       = kReattachStop;
     mChildUpdateAttempts = 0;
     mDataRequestAttempts = 0;
@@ -1667,8 +1671,8 @@ void Mle::HandleAttachTimer(void)
     switch (mAttachState)
     {
     case kAttachStateIdle:
-        OT_ASSERT(false);
-        OT_UNREACHABLE_CODE(break);
+        mAttachCounter = 0;
+        break;
 
     case kAttachStateProcessAnnounce:
         ProcessAnnounce();
@@ -3795,6 +3799,7 @@ void Mle::HandleAnnounce(const Message &aMessage, const Ip6::MessageInfo &aMessa
         mAlternatePanId     = panId;
         SetAttachState(kAttachStateProcessAnnounce);
         mAttachTimer.Start(kAnnounceProcessTimeout);
+        mAttachCounter = 0;
 
         otLogNoteMle("Delay processing Announce - channel %d, panid 0x%02x", channel, panId);
     }

src/core/thread/mle.hpp

@@ -1575,11 +1575,15 @@ private:
         kParentSearchBackoffInterval = (OPENTHREAD_CONFIG_PARENT_SEARCH_BACKOFF_INTERVAL * 1000u),
         kParentSearchJitterInterval  = (15 * 1000u),
         kParentSearchRssThreadhold   = OPENTHREAD_CONFIG_PARENT_SEARCH_RSS_THRESHOLD,
+    };
 
-        // Parameters for "attach backoff" feature (CONFIG_ENABLE_ATTACH_BACKOFF) - Intervals are in milliseconds.
-        kAttachBackoffMinInterval = OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_MINIMUM_INTERVAL,
-        kAttachBackoffMaxInterval = OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_MAXIMUM_INTERVAL,
-        kAttachBackoffJitter      = OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_JITTER_INTERVAL,
+    // Parameters for "attach backoff" feature (CONFIG_ENABLE_ATTACH_BACKOFF) - Intervals are in milliseconds.
+    enum : uint32_t
+    {
+        kAttachBackoffMinInterval         = OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_MINIMUM_INTERVAL,
+        kAttachBackoffMaxInterval         = OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_MAXIMUM_INTERVAL,
+        kAttachBackoffJitter              = OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_JITTER_INTERVAL,
+        kAttachBackoffDelayToResetCounter = OPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_DELAY_TO_RESET_BACKOFF_INTERVAL,
     };
 
     enum ParentRequestType