[mesh-forwarder] inspect forwarded frames to determine if child moved (#2182)

This commit adds logic to inspect frames being forwarded to determine if
a child has moved to a different parent.  If the message was received from
a neighboring router and the IPv6 source address matches that of a child,
the child is assumed to have moved and invalidated.

src/core/thread/mesh_forwarder.cpp

@@ -1859,6 +1859,8 @@ void MeshForwarder::HandleMesh(uint8_t *aFrame, uint8_t aFrameLength, const Mac:
     meshDest.mLength = sizeof(meshDest.mShortAddress);
     meshDest.mShortAddress = meshHeader.GetDestination();
 
+    UpdateRoutes(aFrame, aFrameLength, meshSource, meshDest);
+
     if (meshDest.mShortAddress == netif.GetMac().GetShortAddress())
     {
         aFrame += meshHeader.GetHeaderLength();
@@ -1920,6 +1922,48 @@ exit:
     }
 }
 
+void MeshForwarder::UpdateRoutes(uint8_t *aFrame, uint8_t aFrameLength,
+                                 const Mac::Address &aMeshSource, const Mac::Address &aMeshDest)
+{
+    ThreadNetif &netif = GetNetif();
+    Lowpan::MeshHeader meshHeader;
+    Ip6::Header ip6Header;
+    Neighbor *neighbor;
+
+    VerifyOrExit(meshHeader.Init(aFrame, aFrameLength) == OT_ERROR_NONE);
+
+    // skip mesh header
+    aFrame += meshHeader.GetHeaderLength();
+    aFrameLength -= meshHeader.GetHeaderLength();
+
+    // skip fragment header
+    if (aFrameLength >= 1 &&
+        reinterpret_cast<Lowpan::FragmentHeader *>(aFrame)->IsFragmentHeader())
+    {
+        VerifyOrExit(sizeof(Lowpan::FragmentHeader) <= aFrameLength);
+        VerifyOrExit(reinterpret_cast<Lowpan::FragmentHeader *>(aFrame)->GetDatagramOffset() == 0);
+
+        aFrame += reinterpret_cast<Lowpan::FragmentHeader *>(aFrame)->GetHeaderLength();
+        aFrameLength -= reinterpret_cast<Lowpan::FragmentHeader *>(aFrame)->GetHeaderLength();
+    }
+
+    // only process IPv6 packets
+    VerifyOrExit(aFrameLength >= 1 && Lowpan::Lowpan::IsLowpanHc(aFrame));
+
+    VerifyOrExit(netif.GetLowpan().DecompressBaseHeader(ip6Header, aMeshSource, aMeshDest, aFrame, aFrameLength) > 0);
+
+    neighbor = netif.GetMle().GetNeighbor(ip6Header.GetSource());
+    VerifyOrExit(neighbor != NULL && !neighbor->IsFullThreadDevice());
+
+    if (Mle::Mle::GetRouterId(meshHeader.GetSource()) != Mle::Mle::GetRouterId(GetNetif().GetMac().GetShortAddress()))
+    {
+        netif.GetMle().RemoveNeighbor(*neighbor);
+    }
+
+exit:
+    return;
+}
+
 otError MeshForwarder::CheckReachability(uint8_t *aFrame, uint8_t aFrameLength,
                                          const Mac::Address &aMeshSource, const Mac::Address &aMeshDest)
 {

src/core/thread/mesh_forwarder.hpp

@@ -254,6 +254,8 @@ private:
 
     otError CheckReachability(uint8_t *aFrame, uint8_t aFrameLength,
                               const Mac::Address &aMeshSource, const Mac::Address &aMeshDest);
+    void UpdateRoutes(uint8_t *aFrame, uint8_t aFrameLength,
+                      const Mac::Address &aMeshSource, const Mac::Address &aMeshDest);
 
     otError GetMacDestinationAddress(const Ip6::Address &aIp6Addr, Mac::Address &aMacAddr);
     otError GetMacSourceAddress(const Ip6::Address &aIp6Addr, Mac::Address &aMacAddr);