[mle] fix handling of MLE Orphan Announce messages (#7786)

MLE Orphan Announce messages include a timestamp value that has both
seconds and ticks set to zero, but authoritative bit set. However, the
default Active Timestamp value is all zeros. As a result, an MLE
Orphan Announce timestamp can appear to be greater than the default
Active Timestamp value and cause a device to detach.

This commit adds a special case to check for the Orphan Announce
timestamp.

src/core/meshcop/timestamp.hpp

@@ -140,6 +140,15 @@ public:
      */
     void AdvanceRandomTicks(void);
 
+    /**
+     * This method indicates whether the timestamp indicates an MLE Orphan Announce message.
+     *
+     * @retval TRUE   The timestamp indicates an Orphan Announce message.
+     * @retval FALSE  If the timestamp does not indicate an Orphan Announce message.
+     *
+     */
+    bool IsOrphanTimestamp(void) const { return GetSeconds() == 0 && GetTicks() == 0 && GetAuthoritative(); }
+
     /**
      * This static method compares two timestamps.
      *

src/core/thread/mle.cpp

@@ -4113,7 +4113,15 @@ void Mle::HandleAnnounce(RxInfo &aRxInfo)
 
     localTimestamp = Get<MeshCoP::ActiveDatasetManager>().GetTimestamp();
 
-    if (MeshCoP::Timestamp::Compare(&timestamp, localTimestamp) > 0)
+    if (timestamp.IsOrphanTimestamp() || MeshCoP::Timestamp::Compare(&timestamp, localTimestamp) < 0)
+    {
+        SendAnnounce(channel);
+
+#if OPENTHREAD_CONFIG_MLE_SEND_UNICAST_ANNOUNCE_RESPONSE
+        SendAnnounce(channel, aRxInfo.mMessageInfo.GetPeerAddr());
+#endif
+    }
+    else if (MeshCoP::Timestamp::Compare(&timestamp, localTimestamp) > 0)
     {
         // No action is required if device is detached, and current
         // channel and pan-id match the values from the received MLE
@@ -4136,14 +4144,6 @@ void Mle::HandleAnnounce(RxInfo &aRxInfo)
 
         LogNote("Delay processing Announce - channel %d, panid 0x%02x", channel, panId);
     }
-    else if (MeshCoP::Timestamp::Compare(&timestamp, localTimestamp) < 0)
-    {
-        SendAnnounce(channel);
-
-#if OPENTHREAD_CONFIG_MLE_SEND_UNICAST_ANNOUNCE_RESPONSE
-        SendAnnounce(channel, aRxInfo.mMessageInfo.GetPeerAddr());
-#endif
-    }
     else
     {
         // Timestamps are equal.