[dua] MTD prioritizes DUA in Address Registration TLV (#4862)

Thread 1.2 spec requires prioritizing DUA in Address Registration TLV
to ensure DUA could be stored in the limited addresses space for Child
on the parent.

src/core/backbone_router/leader.hpp

@@ -166,7 +166,7 @@ private:
 #endif
 
     BackboneRouterConfig mConfig;       ///< Primary Backbone Router information.
-    otIp6Prefix          mDomainPrefix; ///< Domain Prefix on the Thread Network.
+    otIp6Prefix          mDomainPrefix; ///< Domain Prefix in the Thread network.
 };
 
 } // namespace BackboneRouter

src/core/thread/dua_manager.cpp

@@ -51,6 +51,7 @@ DuaManager::DuaManager(Instance &aInstance)
 {
     mDomainUnicastAddress.Clear();
     mDomainUnicastAddress.mPreferred          = true;
+    mDomainUnicastAddress.mValid              = true;
     mDomainUnicastAddress.mScopeOverride      = Ip6::Address::kGlobalScope;
     mDomainUnicastAddress.mScopeOverrideValid = true;
 }
@@ -63,6 +64,7 @@ void DuaManager::UpdateDomainUnicastAddress(BackboneRouter::Leader::DomainPrefix
         (aState == BackboneRouter::Leader::kDomainPrefixRefreshed))
     {
         Get<ThreadNetif>().RemoveUnicastAddress(mDomainUnicastAddress);
+        mDomainUnicastAddress.GetAddress().Clear();
     }
 
     VerifyOrExit((aState == BackboneRouter::Leader::kDomainPrefixAdded) ||
@@ -73,17 +75,16 @@ void DuaManager::UpdateDomainUnicastAddress(BackboneRouter::Leader::DomainPrefix
 
     OT_ASSERT(prefix != NULL);
 
-    mDomainUnicastAddress.GetAddress().Clear();
     mDomainUnicastAddress.GetAddress().SetPrefix(prefix->mPrefix.mFields.m8, prefix->mLength);
     mDomainUnicastAddress.mPrefixLength = prefix->mLength;
 
     if (Get<Utils::Slaac>().GenerateIid(mDomainUnicastAddress, NULL, 0, &mDadCounter) == OT_ERROR_NONE)
     {
-        mDomainUnicastAddress.mValid = true;
         Get<ThreadNetif>().AddUnicastAddress(mDomainUnicastAddress);
     }
     else
     {
+        mDomainUnicastAddress.GetAddress().Clear();
         otLogWarnCore("Failed to generate valid DUA");
     }
 

src/core/thread/dua_manager.hpp

@@ -81,6 +81,14 @@ public:
      */
     void UpdateDomainUnicastAddress(BackboneRouter::Leader::DomainPrefixState aState);
 
+    /**
+     * This method returns a reference to the Domain Unicast Address.
+     *
+     * @returns A reference to the Domain Unicast Address.
+     *
+     */
+    const Ip6::Address &GetDomainUnicastAddress(void) const { return mDomainUnicastAddress.GetAddress(); }
+
 private:
     Ip6::NetifUnicastAddress mDomainUnicastAddress;
     uint8_t                  mDadCounter;

src/core/thread/mle.cpp

@@ -1360,6 +1360,25 @@ otError Mle::AppendAddressRegistration(Message &aMessage, AddressRegistrationMod
     VerifyOrExit(aMode != kAppendMeshLocalOnly, OT_NOOP);
     counter++;
 
+#if OPENTHREAD_CONFIG_DUA_ENABLE
+    // Cache Domain Unicast Address.
+    address = Get<DuaManager>().GetDomainUnicastAddress();
+
+    if (Get<ThreadNetif>().IsUnicastAddress(address))
+    {
+        error = Get<NetworkData::Leader>().GetContext(address, context);
+
+        OT_ASSERT(error == OT_ERROR_NONE);
+
+        // Prioritize DUA, compressed entry
+        entry.SetContextId(context.mContextId);
+        entry.SetIid(address.GetIid());
+        SuccessOrExit(error = aMessage.Append(&entry, entry.GetLength()));
+        length += entry.GetLength();
+        counter++;
+    }
+#endif // OPENTHREAD_CONFIG_DUA_ENABLE
+
     for (const Ip6::NetifUnicastAddress *addr = Get<ThreadNetif>().GetUnicastAddresses(); addr; addr = addr->GetNext())
     {
         if (addr->GetAddress().IsLinkLocal() || IsRoutingLocator(addr->GetAddress()) ||
@@ -1368,6 +1387,14 @@ otError Mle::AppendAddressRegistration(Message &aMessage, AddressRegistrationMod
             continue;
         }
 
+#if OPENTHREAD_CONFIG_DUA_ENABLE
+        // Skip DUA that was already appended above.
+        if (addr->GetAddress() == address)
+        {
+            continue;
+        }
+#endif
+
         if (Get<NetworkData::Leader>().GetContext(addr->GetAddress(), context) == OT_ERROR_NONE)
         {
             // compressed entry