Fix crash when combining multicast traffic with VLANs

When IPv4 creates a multicast loopback copy via packet->dup(), the
EncapsulationProtocolReq tag (e.g. ieee8021qctag from PacketTagger) is
inherited. This causes MessageDispatchers to route the loopback copy
through the VLAN/Ethernet path instead of the loopback interface,
leading to a 'MacAddressReq tag is absent' error in
EthernetEncapsulation.

Fix: clear EncapsulationProtocolReq for loopback interfaces in
sendPacketToNIC(), since loopback does not perform link-layer
encapsulation.

Reasoning: IP_MULTICAST_LOOP delivers a local copy as a network-layer
datagram, not as an Ethernet frame. The looped-back copy should go
directly to local IP input and UDP delivery, bypassing link-layer
processing (MAC, VLAN tagging, FCS, etc.) entirely. Only the outward
copy sent via the selected interface should be encapsulated into an
Ethernet frame with a multicast destination MAC.

Author: levy

src/inet/networklayer/ipv4/Ipv4.cc

@@ -1222,7 +1222,9 @@ void Ipv4::sendPacketToNIC(Packet *packet)
     EV_INFO << "Sending " << packet << " to output interface = " << networkInterface->getInterfaceName() << ".\n";
     packet->addTagIfAbsent<PacketProtocolTag>()->setProtocol(&Protocol::ipv4);
     packet->addTagIfAbsent<DispatchProtocolInd>()->setProtocol(&Protocol::ipv4);
-    if (auto networkInterfaceProtocol = networkInterface->getProtocol())
+    if (networkInterface->isLoopback())
+        packet->removeTagIfPresent<EncapsulationProtocolReq>();
+    else if (auto networkInterfaceProtocol = networkInterface->getProtocol())
         ensureEncapsulationProtocolReq(packet, networkInterfaceProtocol, true, false);
     setDispatchProtocol(packet);
     send(packet, "queueOut");