[mle] enhance MTD child IPv6 address registration (openthread#10743)

This commit improves how MTD children register their IPv6 addresses
with their parent. The `Slaac` class now tracks the Lowpan Context
ID (from Network Data) for each SLAAC address. If the Context ID
associated with an existing SLAAC address changes (due to Network
Data updates), the `Slaac` module notifies the `Mle` to schedule
a "Child Update Request" transmission (if the device is an MTD
child). This ensures that the MTD child re-registers its addresses,
resolving any previous registration failures caused by incorrect or
outdated context ID compression.

This commit also adds `test-035-context-id-change-addr-reg.py`
to validate the newly added behavior.

src/core/thread/mle.cpp

@@ -138,8 +138,11 @@ Error Mle::Enable(void)
 
 void Mle::ScheduleChildUpdateRequest(void)
 {
-    mChildUpdateRequestState = kChildUpdateRequestPending;
-    ScheduleMessageTransmissionTimer();
+    if (mChildUpdateRequestState != kChildUpdateRequestPending)
+    {
+        mChildUpdateRequestState = kChildUpdateRequestPending;
+        ScheduleMessageTransmissionTimer();
+    }
 }
 
 Error Mle::Disable(void)
@@ -1055,21 +1058,29 @@ void Mle::InitNeighbor(Neighbor &aNeighbor, const RxInfo &aRxInfo)
     aNeighbor.SetLastHeard(TimerMilli::GetNow());
 }
 
+void Mle::ScheduleChildUpdateRequestIfMtdChild(void)
+{
+    if (IsChild() && !IsFullThreadDevice())
+    {
+        ScheduleChildUpdateRequest();
+    }
+}
+
 void Mle::HandleNotifierEvents(Events aEvents)
 {
     VerifyOrExit(!IsDisabled());
 
     if (aEvents.Contains(kEventThreadRoleChanged))
     {
-        if (IsChild() && !IsFullThreadDevice() && mAddressRegistrationMode == kAppendMeshLocalOnly)
+        if (mAddressRegistrationMode == kAppendMeshLocalOnly)
         {
             // If only mesh-local address was registered in the "Child
             // ID Request" message, after device is attached, trigger a
             // "Child Update Request" to register the remaining
             // addresses.
 
             mAddressRegistrationMode = kAppendAllAddresses;
-            ScheduleChildUpdateRequest();
+            ScheduleChildUpdateRequestIfMtdChild();
         }
     }
 
@@ -1083,10 +1094,7 @@ void Mle::HandleNotifierEvents(Events aEvents)
             Get<Notifier>().Signal(kEventThreadMeshLocalAddrChanged);
         }
 
-        if (IsChild() && !IsFullThreadDevice())
-        {
-            ScheduleChildUpdateRequest();
-        }
+        ScheduleChildUpdateRequestIfMtdChild();
     }
 
     if (aEvents.ContainsAny(kEventIp6MulticastSubscribed | kEventIp6MulticastUnsubscribed))
@@ -1097,15 +1105,14 @@ void Mle::HandleNotifierEvents(Events aEvents)
         // parent of 1.2 or higher version as it could depend on its
         // parent to perform Multicast Listener Report.
 
-        if (IsChild() && !IsFullThreadDevice() &&
-            (!IsRxOnWhenIdle()
+        if (!IsRxOnWhenIdle()
 #if (OPENTHREAD_CONFIG_THREAD_VERSION >= OT_THREAD_VERSION_1_2)
-             || !GetParent().IsThreadVersion1p1()
+            || !GetParent().IsThreadVersion1p1()
 #endif
-                 ))
+        )
 
         {
-            ScheduleChildUpdateRequest();
+            ScheduleChildUpdateRequestIfMtdChild();
         }
     }
 

src/core/thread/mle.hpp

@@ -667,6 +667,15 @@ class Mle : public InstanceLocator, private NonCopyable
         return (&aAddress == &mLinkLocalAllThreadNodes) || (&aAddress == &mRealmLocalAllThreadNodes);
     }
 
+    /**
+     * Schedules a "Child Update Request" transmission if the device is an MTD child.
+     *
+     * For example, the `Slaac` class, which manages SLAAC addresses, calls this method to notify `Mle` that an
+     * existing SLAAC address's Context ID has changed. This can occur due to Network Data updates where the same
+     * on-mesh prefix receives a new Context ID.
+     */
+    void ScheduleChildUpdateRequestIfMtdChild(void);
+
 #if OPENTHREAD_CONFIG_DYNAMIC_STORE_FRAME_AHEAD_COUNTER_ENABLE
     /**
      * Sets the store frame counter ahead.

src/core/utils/slaac_address.cpp

@@ -196,6 +196,15 @@ void Slaac::RemoveOrDeprecateAddresses(void)
             {
                 RemoveAddress(slaacAddr);
             }
+
+            if (UpdateContextIdFor(slaacAddr))
+            {
+                // If the Context ID of an existing address changes,
+                // notify MLE so an MTD child can re-register its
+                // addresses with the parent.
+
+                Get<Mle::Mle>().ScheduleChildUpdateRequestIfMtdChild();
+            }
         }
         else if (!slaacAddr.IsDeprecating())
         {
@@ -341,6 +350,9 @@ void Slaac::AddAddressFor(const NetworkData::OnMeshPrefixConfig &aConfig)
 
     IgnoreError(GenerateIid(*newAddress, dadCounter));
 
+    newAddress->SetContextId(SlaacAddress::kInvalidContextId);
+    UpdateContextIdFor(*newAddress);
+
     LogAddress(kAdding, *newAddress);
 
     Get<ThreadNetif>().AddUnicastAddress(*newAddress);
@@ -349,6 +361,24 @@ void Slaac::AddAddressFor(const NetworkData::OnMeshPrefixConfig &aConfig)
     return;
 }
 
+bool Slaac::UpdateContextIdFor(SlaacAddress &aSlaacAddress)
+{
+    bool            didChange = false;
+    Lowpan::Context context;
+
+    if (Get<NetworkData::Leader>().GetContext(aSlaacAddress.GetAddress(), context) != kErrorNone)
+    {
+        context.mContextId = SlaacAddress::kInvalidContextId;
+    }
+
+    VerifyOrExit(context.mContextId != aSlaacAddress.GetContextId());
+    aSlaacAddress.SetContextId(context.mContextId);
+    didChange = true;
+
+exit:
+    return didChange;
+}
+
 void Slaac::HandleTimer(void)
 {
     NextFireTime nextTime;

src/core/utils/slaac_address.hpp

@@ -168,8 +168,12 @@ class Slaac : public InstanceLocator, private NonCopyable
     class SlaacAddress : public Ip6::Netif::UnicastAddress
     {
     public:
+        static constexpr uint8_t kInvalidContextId = 0;
+
         bool      IsInUse(void) const { return mValid; }
         void      MarkAsNotInUse(void) { mValid = false; }
+        uint8_t   GetContextId(void) const { return mContextId; }
+        void      SetContextId(uint8_t aContextId) { mContextId = aContextId; }
         uint8_t   GetDomainId(void) const { return mDomainId; }
         void      SetDomainId(uint8_t aDomainId) { mDomainId = aDomainId; }
         bool      IsDeprecating(void) const { return (mExpirationTime.GetValue() != kNotDeprecated); };
@@ -188,6 +192,7 @@ class Slaac : public InstanceLocator, private NonCopyable
     private:
         static constexpr uint32_t kNotDeprecated = 0; // Special `mExpirationTime` value to indicate not deprecated.
 
+        uint8_t   mContextId;
         uint8_t   mDomainId;
         TimeMilli mExpirationTime;
     };
@@ -200,6 +205,7 @@ class Slaac : public InstanceLocator, private NonCopyable
     void        DeprecateAddress(SlaacAddress &aAddress);
     void        RemoveAddress(SlaacAddress &aAddress);
     void        AddAddressFor(const NetworkData::OnMeshPrefixConfig &aConfig);
+    bool        UpdateContextIdFor(SlaacAddress &aSlaacAddress);
     void        HandleTimer(void);
     void        GetIidSecretKey(IidSecretKey &aKey) const;
     void        HandleNotifierEvents(Events aEvents);

tests/toranj/cli/cli.py

@@ -351,6 +351,12 @@ def get_pollperiod(self):
     def set_pollperiod(self, period):
         self._cli_no_output('pollperiod', period)
 
+    def get_child_timeout(self):
+        return self._cli_single_output('childtimeout')
+
+    def set_child_timeout(self, timeout):
+        self._cli_no_output('childtimeout', timeout)
+
     def get_partition_id(self):
         return self._cli_single_output('partitionid')
 
@@ -368,6 +374,9 @@ def get_parent_info(self):
     def get_child_table(self):
         return Node.parse_table(self.cli('child table'))
 
+    def get_child_ip(self):
+        return self.cli('childip')
+
     def get_neighbor_table(self):
         return Node.parse_table(self.cli('neighbor table'))