[mesh-forwarder] simplify multicast forward to sleepy children (opent…

…hread#9505)

This commit simplifies the `MeshForwarder::SendMessage()` method and
its logic for determining whether to send a multicast message to
sleepy children. We first determine whether the destination address
is link-local or realm-local all-nodes, meaning it is destined to all
nodes. Otherwise, we need to check whether each sleepy child is
subscribed to the address. We then iterate through the child table
and mark the message for indirect transmission accordingly.

src/core/thread/mesh_forwarder_ftd.cpp

@@ -48,8 +48,7 @@ RegisterLogModule("MeshForwarder");
 
 Error MeshForwarder::SendMessage(Message &aMessage)
 {
-    Mle::MleRouter &mle   = Get<Mle::MleRouter>();
-    Error           error = kErrorNone;
+    Error error = kErrorNone;
 
     aMessage.SetOffset(0);
     aMessage.SetDatagramTag(0);
@@ -60,57 +59,48 @@ Error MeshForwarder::SendMessage(Message &aMessage)
     {
     case Message::kTypeIp6:
     {
-        Ip6::Header ip6Header;
+        Ip6::Header         ip6Header;
+        const Ip6::Address &destination = ip6Header.GetDestination();
 
         IgnoreError(aMessage.Read(0, ip6Header));
 
-        if (ip6Header.GetDestination().IsMulticast())
+        if (destination.IsMulticast())
         {
             // For traffic destined to multicast address larger than realm local, generally it uses IP-in-IP
             // encapsulation (RFC2473), with outer destination as ALL_MPL_FORWARDERS. So here if the destination
             // is multicast address larger than realm local, it should be for indirection transmission for the
             // device's sleepy child, thus there should be no direct transmission.
-            if (!ip6Header.GetDestination().IsMulticastLargerThanRealmLocal())
+            if (!destination.IsMulticastLargerThanRealmLocal())
             {
-                // schedule direct transmission
                 aMessage.SetDirectTransmission();
             }
 
             if (aMessage.GetSubType() != Message::kSubTypeMplRetransmission)
             {
-                if (ip6Header.GetDestination() == mle.GetLinkLocalAllThreadNodesAddress() ||
-                    ip6Header.GetDestination() == mle.GetRealmLocalAllThreadNodesAddress())
-                {
-                    // destined for all sleepy children
-                    for (Child &child : Get<ChildTable>().Iterate(Child::kInStateValidOrRestoring))
-                    {
-                        if (!child.IsRxOnWhenIdle())
-                        {
-                            mIndirectSender.AddMessageForSleepyChild(aMessage, child);
-                        }
-                    }
-                }
-                else
+                // Check if we need to forward the multicast message
+                // to any sleepy child. This is skipped for MPL retx
+                // (only the first MPL transmission is forwarded to
+                // sleepy children).
+
+                bool destinedForAll = ((destination == Get<Mle::Mle>().GetLinkLocalAllThreadNodesAddress()) ||
+                                       (destination == Get<Mle::Mle>().GetRealmLocalAllThreadNodesAddress()));
+
+                for (Child &child : Get<ChildTable>().Iterate(Child::kInStateValidOrRestoring))
                 {
-                    // destined for some sleepy children which subscribed the multicast address.
-                    for (Child &child : Get<ChildTable>().Iterate(Child::kInStateValidOrRestoring))
+                    if (!child.IsRxOnWhenIdle() && (destinedForAll || child.HasIp6Address(destination)))
                     {
-                        if (!child.IsRxOnWhenIdle() && child.HasIp6Address(ip6Header.GetDestination()))
-                        {
-                            mIndirectSender.AddMessageForSleepyChild(aMessage, child);
-                        }
+                        mIndirectSender.AddMessageForSleepyChild(aMessage, child);
                     }
                 }
             }
         }
         else // Destination is unicast
         {
-            Neighbor *neighbor = Get<NeighborTable>().FindNeighbor(ip6Header.GetDestination());
+            Neighbor *neighbor = Get<NeighborTable>().FindNeighbor(destination);
 
             if ((neighbor != nullptr) && !neighbor->IsRxOnWhenIdle() && !aMessage.IsDirectTransmission() &&
                 Get<ChildTable>().Contains(*neighbor))
             {
-                // Destined for a sleepy child
                 mIndirectSender.AddMessageForSleepyChild(aMessage, *static_cast<Child *>(neighbor));
             }
             else