[mesh-forwarder] skip mesh header immediately after parsing it (#4453)

This commit updates `HandleMesh()` method to skip the mesh header in
the frame immediately after parsing it. This in turn removes the need
for other methods (`CheckReachability` or `UpdateRoutes()` calling
through `GetIp6Header()` to `DecompressIp6Header()`) to redo the
parsing to skip the mesh header. With this change we can also remove
the now unused `SkipMeshHeader()`.

src/core/thread/mesh_forwarder.cpp

@@ -389,21 +389,6 @@ void MeshForwarder::GetMacDestinationAddress(const Ip6::Address &aIp6Addr, Mac::
     }
 }
 
-otError MeshForwarder::SkipMeshHeader(const uint8_t *&aFrame, uint16_t &aFrameLength)
-{
-    otError            error = OT_ERROR_NONE;
-    Lowpan::MeshHeader meshHeader;
-    uint16_t           headerLength;
-
-    VerifyOrExit(Lowpan::MeshHeader::IsMeshHeader(aFrame, aFrameLength));
-    SuccessOrExit(error = meshHeader.ParseFrom(aFrame, aFrameLength, headerLength));
-    aFrame += headerLength;
-    aFrameLength -= headerLength;
-
-exit:
-    return error;
-}
-
 otError MeshForwarder::GetFragmentHeader(const uint8_t *         aFrame,
                                          uint16_t                aFrameLength,
                                          Lowpan::FragmentHeader &aFragmentHeader)
@@ -432,8 +417,6 @@ otError MeshForwarder::DecompressIp6Header(const uint8_t *     aFrame,
     Lowpan::FragmentHeader fragmentHeader;
     int                    headerLength;
 
-    SuccessOrExit(error = SkipMeshHeader(aFrame, aFrameLength));
-
     if (GetFragmentHeader(aFrame, aFrameLength, fragmentHeader) == OT_ERROR_NONE)
     {
         // only the first fragment header is followed by a LOWPAN_IPHC header

src/core/thread/mesh_forwarder.hpp

@@ -345,7 +345,6 @@ private:
                          const Mac::Address &aMeshSource,
                          const Mac::Address &aMeshDest);
 
-    otError  SkipMeshHeader(const uint8_t *&aFrame, uint16_t &aFrameLength);
     otError  DecompressIp6Header(const uint8_t *     aFrame,
                                  uint16_t            aFrameLength,
                                  const Mac::Address &aMacSource,

src/core/thread/mesh_forwarder_ftd.cpp

@@ -516,14 +516,14 @@ void MeshForwarder::HandleMesh(uint8_t *               aFrame,
     meshSource.SetShort(meshHeader.GetSource());
     meshDest.SetShort(meshHeader.GetDestination());
 
+    aFrame += headerLength;
+    aFrameLength -= headerLength;
+
     UpdateRoutes(aFrame, aFrameLength, meshSource, meshDest);
 
     if (meshDest.GetShort() == Get<Mac::Mac>().GetShortAddress() ||
         Get<Mle::MleRouter>().IsMinimalChild(meshDest.GetShort()))
     {
-        aFrame += headerLength;
-        aFrameLength -= headerLength;
-
         if (reinterpret_cast<Lowpan::FragmentHeader *>(aFrame)->IsFragmentHeader())
         {
             HandleFragment(aFrame, aFrameLength, meshSource, meshDest, aLinkInfo);
@@ -548,9 +548,6 @@ void MeshForwarder::HandleMesh(uint8_t *               aFrame,
 
         meshHeader.DecrementHopsLeft();
 
-        aFrame += headerLength;
-        aFrameLength -= headerLength;
-
         GetForwardFramePriority(aFrame, aFrameLength, meshSource, meshDest, priority);
         message = Get<MessagePool>().New(Message::kType6lowpan, priority);
         VerifyOrExit(message != NULL, error = OT_ERROR_NO_BUFS);